# Engaging students: Solving word problems of the form “a is p% of b”

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission again comes from my former student Christian Oropeza. His topic, from Algebra: solving word problems of the form “a is p% of b.”

What interesting (i.e., uncontrived) word problems using this topic can your students do now? (You may find resources such as http://www.spacemath.nasa.gov to be very helpful in this regard; feel free to suggest others.)

Students would be able to answer word problems that involve real world applications. For example, a student could be asked: “Sam went to Academy to buy clothes, sports equipment, and fishing gear. At the register the total of Sam’s transaction before tax is \$141.32. Given that the sales tax is 8.25%, what would Sam’s total be after tax?” These type of word problems would be relatable to students, which would show them the importance of this topic in life. Students always ask the question, “how is this used in everyday life?”, and with these type of word problems students may be able to generalize the concept more easily. When students cannot relate to a topic in math they become easily discouraged, give up, and stop paying attention in class, but with problems like these the students would be able to incorporate the topic into their own lives. Some other problems that students could be asked could involve any type of scenario where there is a percentage to be found between two numbers (Reference 1 & 4).

How can this topic be used in your students’ future courses in mathematics or science?

This topic can be used in different scenarios for math and science, but Chemistry is an excellent example. In chemistry, there is a topic that covers calculating percent composition. The basic idea of this topic is to calculate the percentage of each element’s mass in regard to a molecule’s total molecular mass. An example would be, “Calculate the mass percent composition of each element in a potassium ferricyanide, K3Fe(CN)6 molecule.” (Reference 2). These types of problems would help students understand how much a certain element or compound is in a particular molecule. Another example of how this topic can be used, is in math when a student has to convert between fractions, decimals, and percentages in a word problem. An example could be, “Mia has a basket full of fruit. In this basket she has 1/5 apples, 2/3 oranges, and 2/15 bananas. What percent of each fruit does she have in relation to the basket?” Students would be able to work on their converting skills to enhance their understanding of multiple representations of the same number (Reference 3).

How can technology (YouTube, Khan Academy [khanacademy.org], Vi Hart, Geometers Sketchpad, graphing calculators, etc.) be used to effectively engage students with this topic? Note: It’s not enough to say “such-and-such is a great website”; you need to explain in some detail why it’s a great website.

Technology is always a great way to engage students especially with the newer generation of students where technology is part of their everyday life. The website mathisfun.com (Reference 4) is an excellent piece of technology to introduce or review this topic to the students because the website goes through visual representations of how a percentage of a whole looks like. Also, the website has a section where a student can input a number and a slider that allows the student to move it around to see what number would represent a certain percentage of the number inputted. Another example of effective technology is the website Khan Academy (Reference 1) because it has real world problems that are relatable. The website also gives hints and step-by-step solutions for each question in case a student is stuck and does not know what to do next. The use of multiple websites is good for students to have a variety to choose from in case one is easier to understand than another.

References:

# Engaging students: Box and whisker plots

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This submission comes from my former student Chris Brown. His topic: how to engage students when teaching box and whisker plots.

How could you as a teacher create an activity or project that involves your topic?

My all-time favorite TV show as a child was Pokémon. This show is still a staple amongst the young and even adult generation of today. The activity that I have created, was designed to take place after a formal lesson over how to create Box and Whisker plots. For this activity, students will be given a labeled bar graph of the Pokémon Type Distribution for generations 1 through 6 of Pokémon, which I have listed an online data source below. The students will be tasked with identifying the top 7 Pokémon types and creating a Box and Whiskers plots for each of those types. They will then go through and analyze the consistency of the creation of Pokémon for that specific type and then compare contrast this same box plot to any other box plot of their choice. The students will then make predications for the number of Pokémon for each of the top 7 Pokémon types, for generation 7 and base their reasoning in the box plots they created. Then the student will finally research the type distributions for the 7th generation of Pokémon, and discuss how the actual number compares to their prediction.

This is the online source for the type distributions for generations 1 – 6:

https://plot.ly/~powersurge360/6.embed

How does this topic extend what your students should have learned in previous courses?

From my experience, Box Plots are first taught in the early middle school years, in 6th or 7th grade. When constructing box plots by hand, in its essence, box plots require knowledge of how to order sets of numbers from least to greatest; an understanding and ability to find the maximum, minimum, median, and mean of a data set; and lastly, critical thinking and analytic skills developed from general course content. Box plots allow students to combine each of these skills to effectively analyze data sets with ease and compare different data sets with precision and accuracy. If any or all of these skills are not quite up to par, students will have an opportunity to develop them through box plots as they spend time creating them. For all students no matter their level, they will still gain better insight on how to properly analyze data and grow as analytical thinkers as they take the represented data and turn it into meaningful interpretations.

How can technology be used to effectively engage students with this topic?

In a classroom, I personally believe that Desmos is a wonderful online tool that can aid students in the understanding of how box and whisker plots function, and also a great place to check their work. Desmos, which is linked below, gives students the ability to list as many data points as they need to, and concurrently creates a box plot as they do so. In this way, students are able to see how singular data points can skew the data in significant and insignificant amounts. What I also love about Desmos is that, the list of data points does not have to be in any kind of order, so students do not have to worry about that tedious step! Desmos also lists the 5-point summary in two different places, on the box plot itself, and also on a drop-down menu, which is super convenient. Lastly, I love how Desmos also displays the mean of the data set as well, students can calculate the skew of the data, and definitively determine how it is skewed. This is a super visual, and interactive tool that will allow the student to manipulate box plots so seamlessly they will not be focused on the tediousness of the setup and solely on the concept.

The link to the Desmos setup is here: https://www.desmos.com/calculator/h9icuu58wn

# Engaging students: Ratios and rates of change

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission again comes from my former student Cameron Story. His topic, from Algebra: ratios and rates of change.

What interesting word problems using this topic can your students do now?

Since the most relatable example of a ratio is speed (meters per second, miles per hour, etc.), it’s easy to see how a teacher can make an interesting or engaging word problem out of this. First, however, let us take a look at an infamous word problem involving ratios/rates of change that is not inherently interesting on its own.

“Train A, traveling 70 miles per hour (mph), leaves Westford heading toward Eastford, 260 miles away. At the same time Train B, traveling 60 mph, leaves Eastford heading toward Westford. When do the two trains meet? How far from each city do they meet?” (“The Two Trains.” Mathforum.org, National Council of Teachers of Mathematics, mathforum.org/dr.math/faq/faq.two.trains.html.)

This is a distance-over-time that most students or past students are familiar with, but why is this problem still being used? There are a few issues I have with this example. Firstly, I cannot think of very many students who could honestly get excited about trains, especially now in the modern era of vehicular travel. I am willing to bet that most of your high school math students have never even been on a train; and if they have, it was most likely an underwhelming experience. This example also lacks creativity. Giving the trains actual names or having them traveling between real world places would have been a step in the right direction.

So how can we change this example to become engaging to students? Firstly, let’s replace the trains with modern cars, and crank up the speed. Every student is familiar with cars, and fast-moving cars (in my opinion) is much more exciting. One could easily imagine using modern rockets as the vehicle as well, and replacing the towns with interplanetary destinations. Next, instead of naming the cars Car A and Car B, we can use actual modern electric cars such as the Model 3 from Tesla Motors. Take a look of the following word problem I came up with instead (you may notice the stakes of the situation described is objectively more engaging then a problem about train travel):

“Tesla is hoping to feature one of its new cars in a commercial, in which a car attempts to race underneath a falling refrigerator in dramatic fashion. In the commercial, the car must travel at top speed, traveling over 25 meters of track from start to finish. As soon as the car passes the starting line, the fridge is dropped from 10 meters up in the air above the finish line, at a rate of 20 meters per second. The top speeds (in meters per second) of the Tesla Model 3 and the Tesla Roadster are shown below. Which car should Tesla pick to safely beat the falling fridge?”

The reason a creative approach works better is that it increases the student engagement; students do not want to do word problems, so it is our job as teachers to make them interesting.

How could you as a teacher create an activity or project that involves your topic?

Creating an activity around rates of change allows for a lot of creativity. For example, one could take a physical approach, in which students record how fast they can run (only requires a stop watch and a set distance) and using that to plot their data on a distance vs. time graph.

It is important to remember that ratios can represent far more than just speed. Some relatable examples include rate of hair growth, number of hours studied per week, or even how many gallons of water drank in a day. For my Tesla commercial word problem, I used a website (desmos.com) to flesh out this one problem into an engaging classroom activity. Having your classroom activities on interactive platforms that evoke teamwork and cooperation in your students is key to student engagement.

How can technology be used to effectively engage students with this topic?

Desmos Classroom Activities (at teacher.desmos.com) is an incredibly useful tool that teachers can use to quickly create any activity for their students. These activities can even be done on smart phones, which removes some of the hassle of getting computers in the classroom. When creating an activity, teachers also have access to a wide range of tools including (but not limited to) animation, student inputs, information slides (for presentation), and even interactive functions that allow students to modify given equations.

The main benefit of using Desmos for classroom activities is that the teacher has full and complete access to viewing student progress. Instead of walking around the room trying to hunt down students who need help, the teacher can view which students are stuck on which problems. The teacher can then approach the issue fully prepared, and know exactly which students are having problems before their hands even hit the air.

I created a Desmos activity available for use in a lesson about ratios or rates of change (link: https://teacher.desmos.com/activitybuilder/custom/5b887ad92c2ff330af6b87c0) which uses the same Tesla commercial word problem I gave before. Using this website, I was able to build this world problem into a somewhat-realistic and animated simulation, asking critical questions in order to build upon the underlying mathematical concepts. Feel free to adapt my lesson (Desmos has a copy/edit feature for activities) for any vehicle, scenario, or speed.

References:

“Desmos Classroom Activities.” Desmos Classroom Activities, 2010, teacher.desmos.com/.

Story, Cameron. “Ratios and Rates of Change Activity.” Desmos Classroom Activities, 30 Aug. 2018, teacher.desmos.com/activitybuilder/custom/5b887ad92c2ff330af6b87c0.

“The Two Trains.” Mathforum.org, National Council of Teachers of Mathematics, mathforum.org/dr.math/faq/faq.two.trains.html.

# Engaging students: Arithmetic sequences

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission comes from my former student Danielle Pope. Her topic, from Precalculus: arithmetic sequences.

How can this topic be used in your students’ future courses in mathematics or science?

In the future, the topic of arithmetic sequences will be built upon by introducing another sequence, the geometric sequence. A geometric sequence is just a sequence of multiples instead of increasing by a constant. The next topic introduced will be finding the sum of a sequence of numbers. This will be introduced as a series. The summation symbol will also be introduced to kids and they will learn that new notation. Summations will bring along many formulas for finding the leading coefficient and will show up later in Calculus 2 classes when talking about convergence and divergence of series. Another one of the things that kids will always be doing with sequences and series is finding the general form of a given sequence or series. Through school, this idea will never change the sequence and series will just get harder to identify.

What interesting things can you say about the people who contributed to the discovery and/or the development of this topic? (You might want to consult Math Through The Ages.)

An arithmetic sequence is a set of numbers that have a constant difference between each term. One of the main people that come up when researching these sequences is Carl Friedrich Gauss. Many math-loving people know him as the “Prince of mathematicians”. He is famous for coming up with the equations to solve the sum of an arithmetic sequence. This comes as no surprise that he came up with this formula. The surprising thing about this realization is that he made it at an age young enough to still be in grade school. Stories say that Gauss was asked to solve for the sum on the board in grade school and used the formula of M ( M + 1 ) / 2 to solve for the correct answer. This just goes to show that anyone can, in fact, contribute to the greater good of mathematics at any age.

How have different cultures throughout time used this topic in their society?

One of the first civilizations that utilized sequences was the Egyptians. They used the sequence of multiples of 2 to do their multiplication. The basic sequence is 1, 2, 4, 8, 16, 32, … and we are trying to solve 24 x 13 with the process pictured below.

The process behind this is to write the multiple of 2 sequences down the left side of the paper until you reach the largest multiple of 2 without going over the second number being multiplied, in this case, 13. Once that is done set the first term on the right side equal to the first number being multiplied, in this case, 24. Next, multiply the right side by 4 until you get the same amount of terms on the left side. Lastly find the sum of numbers on the left that add to 13, which are 1, 4, and 8. Add the corresponding multiples from the side, 24 + 96 + 192 = 312. The right side sum of the corresponding numbers checked on the left gives the product of the original problem, i.e. 312. This trick is cool to show just on its own but it’s also cool because it uses something as simple as a specific list of numbers aka a sequence of numbers.

References

http://www.softschools.com/facts/scientists/carl_friedrich_gauss_facts/827/

https://rabungapalgebraiii.wikispaces.com/Arithmetic+Sequences+and+Series

http://www.math.wichita.edu/MathCircle/docs/egyptian_arithmetic.pdf

# Engaging students: Introducing the number e

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission comes from my former student Deanna Cravens. Her topic, from Precalculus: introducing the number $e$.

The number e is a relatively newer irrational number if compared to pi. However, it first made its appearance very subtly in 1618. Napier was working on a table of natural logarithms, however it was not noted that the base was e. There were a few other appearances of e but mathematicians had not truly made a connection to it. Eventually in 1683, Jacob Bernoulli was looking at a business application dealing with continuously compounded interest and recognized that the log function and the exponential function were inverses. In 1690, a letter was written by Leibniz and e officially had a name, except it was called ‘b’ at the time. As it comes to no surprise, Euler had his hand in discovering e. He published Introductio in Analysin infinitorum in 1748 where he showed that e is the limit of $(1 + 1/n)^n$. Now Euler did not explicitly prove that e is irrational, however most people accepted it at that point, but it was indeed later proven.

How could you as a teacher create an activity or project that involves your topic?
Where does the number e come from? Well, the answer is a business application dealing with continuously compounded interest. However, students in a pre-calculus class can easily discover the number e without having to use the calculus behind it. Simply give students this short activity at the beginning of class.

One of the good things about this activity is that it gives a brief snippet of the history of e before students begin to calculate it. Then, students can easily use a calculator and plug in the listed values in the table into the equation $(1+1/n)^n$. As the numbers get increasingly large, students will notice that they will all appear to be getting closer to 2.718… which is now known as the number e. As a teacher it is important to note that e is like pi, it is an irrational number that goes on forever and doesn’t have any sort of repeating pattern, yet it is extremely important in mathematics.

How can technology (YouTube, Khan Academy [khanacademy.org], Vi Hart, Geometers Sketchpad, graphing calculators, etc.) be used to effectively engage students with this topic?

This video would be excellent to show students who are asking, “why is e so important or where does it come from?” The video starts out by stating what e is approximately equal to. Then it gives a brief history about e and talks about compounded interest. It does a great job at explaining compounded interest. It is executed in a way where pre-calculus students can easily understand the concept. It also uses good visual cues to show how it would work. Next it lists several applications of e. These applications include: statistics through the normal curve, biology by modeling population growth, and physics by the exponential decay of a radioactive material. Overall, it does a great job showing the importance of e in real world applications. Thus, showing the importance of e to a pre-calculus students.

# Engaging students: Synthetic Division

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission comes from my former student Deetria Bowser. Her topic, from Precalculus: synthetic division.

How can technology (YouTube, Khan Academy [khanacademy.org], Vi Hart, Geometers Sketchpad, graphing calculators, etc.) be used to effectively engage students with this topic? Note: It’s not enough to say “such-and-such is a great website”; you need to explain in some detail why it’s a great website.

It is tough to find websites or technology that help with synthetic division, due to the fact that most websites consists of a long list of instruction, which is not engaging. One website that does seem helpful to a student learning synthetic division is: http://emathlab.com. Under the Algebra tab one can select the option for polynomial functions, then synthetic division. Once selected, a synthetic division activity will pop up. In this activity the student is given a polynomial (of third degree most of the time) divided by a degree one polynomial. The student is then expected to correctly fill the cells with the correct numbers for synthetic division. If they do not get the correct number, the cell turns red and they have to keep trying until they get the answer correct. This activity will be beneficial to students because they will be able to get a feel on the correct placement of numbers when using synthetic division. Additionally, this tool will help them realize what to do when they get polynomials, such as $x^3-1$. Finally this online tool will allow the students to evaluate themselves.

How can this topic be used in your students’ future courses in mathematics or science?

The idea of synthetic division is used to find the zeros of a function. One may need to find zeros of a function in a variety of mathematics and science courses. For example, in physics, one may need to find the roots of a trajectory equation. To find said roots, one could use synthetic division. Also an example of finding roots could be used to help in computer programming. On math.stackexchange.com a programming student presents the following problem: “I am currently programming a simulation for a pinball game and want to calculate the time when the ball hits a circle (if they collide at some point). For the calculation part, I’m adding the radius of the ball to the radius of the circle, so that i only have to check if the midpoint of the ball collides with the circle. Of course, the circle is displayed with it’s original radius.

Now for the ball’s (midpoint) trajectory i’ve got these two equations who define the movement of the ball on the x- and y-axis (depending on the gravitational acceleration):

$x(t)=s_x+v_x t, \quad y(t)=s_y+v_y t− \frac{1}{2} g t^2$,

with $(s_x,s_y)$= starting point of the ball, $(v_x,v_y)$= initial velocity, $g$= gravitational acceleration and $t$= time.

To check for collision, I took these two equations and put them into the equation of a circle. Once multiplied out the student got something of the form: $a t^4+bt^3+ct^2+dt+e=0$. If the coefficients $a,b,c,d,e$ are rational numbers, then he will be able to use synthetic division to find all of the roots, and successfully create his game.

How does this topic extend what your students should have learned in previous courses?

In previous courses, students are taught to find zeros by either graphing, guessing and plugging in a number for x and hoping that the result is zero, or using long division. Synthetic division provides a more systematic way of finding zero’s than just guessing, and can prove to be quicker than graphing and using long division. Additionally, synthetic division can expand on the idea of showing something is not a factor. For instance, when one tries to synthetically divide the polynomial $x^4-3x^2+5x-7$ by $x-2$ one will get a remainder of 7. This is another way of proving that $x-2$ is not a factor of $x^4-3x^2+5x-7$. Also, one now knows what the polynomial $x^4-3x2+5x-7$ is when $x = 2$. Synthetic division, extends the idea of finding factors and non-factors of polynomials, as well as solutions to polynomials at a specific $x$.

References

# Statistics for People in a Hurry

The following article was recommended to me by a former student: https://towardsdatascience.com/statistics-for-people-in-a-hurry-a9613c0ed0b. It’s synopsis is in the opening paragraph:

Ever wished someone would just tell you what the point of statistics is and what the jargon means in plain English? Let me try to grant that wish for you! I’ll zoom through all the biggest ideas in statistics in 8 minutes! Or just 1 minute, if you stick to the large font bits.

# Engaging students: Computing the composition of two functions

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission again comes from my former student Alexandria Johnson. Her topic, from Algebra II/Precalculus: computing the composition of two functions.

The following link is to a worksheet over composition of functions. The worksheet allows students to explore composition of functions without outright telling them what composition of functions is. Instead, the students are working on real world problems about shopping in a store that is having a 20% sale with mystery coupons. In the worksheet, students explore whether or not it matters which discount is applied first and the equations that go along with each scenario. This worksheet is interesting because it approaches composition of functions in an explorative way and it is using a real-world situation students in high school may find relatable, which can help hook students that are math-phobic.

https://betterlesson.com/community/document/1326462/going-shopping-student-materials-docx

Computing the composition of two functions requires prior knowledge of basic operations and combining like terms. This topic will expand upon their knowledge of basic operations by applying them to functions. Students will be able to add, subtract, multiply, and divide functions. Students should be able to use the distribution property; this is important when students are writing (fog)(x) and (gof)(x). During this topic, students should be able to expand upon their knowledge of creating functions from real world problems, which can be seen in the worksheet from the link above.

Musical composition is a way this topic can appear in high culture. Musical composition is the process of combining notes, chords, and melodies in a particular way. Arranging the notes, chords, or melodies in different ways can change the composition. Function composition is the combining of different functions f(x) and g(x) in different ways like addition, subtraction, multiplication, and division. Order usually matters in function composition just like in musical composition. If you have several band students, or musically inclined students, this would be a good hook to grab students interest.

# Engaging students: Using a recursively defined sequence

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission comes from my former student Austin DeLoach. His topic, from Precalculus: using a recursively defined sequence.

How could you as a teacher create an activity or project that involves your topic?
One activity that would be interesting to introduce recursion would be Fibonacci’s rabbit problem. In his book, Liber Abaci, Fibonacci introduced a problem where you start with one young pair of rabbits and try to find out how many rabbits you would have after a year. Every month, a grown pair of rabbits can give birth to a new pair, and it only takes one month for a young pair to grow up and be able to reproduce on their own, and the rabbits also never die. This is one of the most popular recursive sequences (the Fibonacci sequence), and, by itself, can be solved without a prior knowledge of recursion, but is a very good way to introduce the idea once the students begin to analyze the pattern of how many pairs of rabbits there are after each month. This problem is laid out in this video, https://youtu.be/sjQlW6cH3Ko but it is not necessary to show the video to introduce the problem.

How can this topic be used in your students’ future courses in mathematics or science?

One major place that a solid grasp of recursion can be used is in computer programming courses. Although not everyone takes these, they are becoming increasingly popular and the field is not likely to shrink any time soon. In programming, there are certain things that can either only be written recursively (as opposed to explicitly) or at least ones that are simpler to write and understand with recursion than with an explicit algorithm. There are also times, depending on the language and content, that a recursive function can be more efficient. Because of this, an understanding of recursion is becoming increasingly important for more people, and the ability to write and understand how it works is practically becoming necessary. So, even though not every student will go on to take computer science, many will, and the basic idea is still important to understand.

How can technology be used to effectively engage students with this topic?

There is a series of Khan Academy videos on recursively defined sequences online. The first one is https://youtu.be/lBtb30SjU2Q and it shows how to read and understand what the basic frame for recursion is. Although Khan Academy videos are not always the most engaging for all students, they do work for many because of their consistent structure. This video in particular is about recursive formulas for arithmetic sequences. Without mentioning the vocabulary yet, the video does introduce the idea of a base case and the method for finding subsequent values. The video both shows how to look at a list of values and determine the recursive definition, as well as how to understand the recursive definition if that is what you are given. For a three minute video, it does a very good job of introducing important topics for recursive series and explaining the basic ideas so that students have a framework to build on later when more complex recursively defined sequences are introduced.

# Engaging students: Finding the focus and directrix of a parabola

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission comes from my former student Brittnee Lein. Her topic, from Precalculus: finding the focus and directrix of a parabola.

What are the contributions of various cultures to this topic?

Parabolas (as we know them) were first written about in Apollonius’s Conics. Apollonius stated that parabolas were the result of a plane cutting a double right circular cone at an angle parallel to the vertical angle (α). So, what does that actually mean?

Well, if we take a vertical line and intersect it with a straight line at a fixed point, and then rotate that straight line around the fixed point we form the shape below:

If the plane slices the cone at the angle β and β=α, a parabola is formed. This is still how we define parabolas today although you may not think about it that way. When you think of a parabola, you think of the equation $y = ax^2 +bx + c$. This equation is derived using the focus and the directrix. This video shows how to do so:

Understanding how the focus and directrix affect the equation of a parabola is crucial to understanding what each word means. According to mathwords.com, “For a given point, called the focus, and a given line not through the focus, called the directrix, a parabola is the locus of points such that the distance to the focus equals the distance to the directrix.” The directrix is a line perpendicular to the axis of symmetry and the focus falls on the line of the axis of symmetry.

How can technology be used to effectively engage students with this topic?

This desmos activity can be used to show students how changing the focus, directrix, and vertex of the parabola affects the graph. https://www.desmos.com/calculator/y90ffrzmco

From this, students can shift values of the vertex and see that the directrix stays constant when the x-value is changed and that the focus remains constant when the y-value is shifted. If students change the value of the focus, they can see how it stretches and contracts the width of the parabola and how the directrix shifts. They can also see that when the focus is negative, the parabola opens downward and the directrix is positive. This website: https://www.intmath.com/plane-analytic-geometry/parabola-interactive.php Is also very helpful in showing the relationships between the focus, directrix and the graph of the parabolas because students can clearly see that the distance between a point on the parabola and the focus and the distance between that same point and the directrix are equal.

What interesting (i.e., uncontrived) word problems using this topic can your students do now?

The website http://www.purplemath.com/modules/parabola4.htm has a lot of great real-world word problems involving finding the focus and the directrix of a parabola. For example, one of the questions is:

(This is a graph I made using desmos to model the situation at hand)

This problem requires a lot of prior knowledge of parabolas and really tests students’ ability to interpret information. From the question alone, the students can find the x-intercepts (-15,0) and (15,0) from the information “the base has a width of 30 feet”. They are also able to infer that the slope of the parabola will be negative because of the shape of an arch. The student must also know how to find the slope of the parabola using the x-intercepts, solving for the equation of the parabola using the x-intercepts and vertex and the equations for finding the focus and directrix from the given information. There are a few problems as involved as this one on the listed website above.