# My Favorite One-Liners: Part 88

In this series, I’m compiling some of the quips and one-liners that I’ll use with my students to hopefully make my lessons more memorable for them.

In the first few weeks of my calculus class, after introducing the definition of a derivative,

$\displaystyle \frac{dy}{dx} = y' = f'(x) = \lim_{h \to 0} \displaystyle \frac{f(x+h) - f(x)}{h}$,

I’ll use the following steps to guide my students to find the derivatives of polynomials.

1. If $f(x) = c$, a constant, then $\displaystyle \frac{d}{dx} (c) = 0$.
2. If $f(x)$ and $g(x)$ are both differentiable, then $(f+g)'(x) = f'(x) + g'(x)$.
3.  If $f(x)$ is differentiable and $c$ is a constant, then $(cf)'(x) = c f'(x)$.
4. If $f(x) = x^n$, where $n$ is a nonnegative integer, then $f'(x) = n x^{n-1}$.
5. If $f(x) = a_n x^n + a_{n-1} x^{n-1} + \dots + a_1 x + a_0$ is a polynomial, then $f'(x) = n a_n x^{n-1} + (n-1) a_{n-1} x^{n-2} + a_1$.

After doing a few examples to help these concepts sink in, I’ll show the following two examples with about 3-4 minutes left in class.

Example 1. Let $A(r) = \pi r^2$. Notice I’ve changed the variable from $x$ to $r$, but that’s OK. Does this remind you of anything? (Students answer: the area of a circle.)

What’s the derivative? Remember, $\pi$ is just a constant. So $A'(r) = \pi \cdot 2r = 2\pi r$.

Does this remind you of anything? (Students answer: Whoa… the circumference of a circle.)

Generally, students start waking up even though it’s near the end of class. I continue:

Example 2. Now let’s try $V(r) = \displaystyle \frac{4}{3} \pi r^3$. Does this remind you of anything? (Students answer: the volume of a sphere.)

What’s the derivative? Again, $\displaystyle \frac{4}{3} \pi$ is just a constant. So $V'(r) = \displaystyle \frac{4}{3} \pi \cdot 3r^2 = 4\pi r^2$.

Does this remind you of anything? (Students answer: Whoa… the surface area of a sphere.)

By now, I’ve really got my students’ attention with this unexpected connection between these formulas from high school geometry. If I’ve timed things right, I’ll say the following with about 30-60 seconds left in class:

Hmmm. That’s interesting. The derivative of the area of a circle is the circumference of the circle, and the derivative of the area of a sphere is the surface area of the sphere. I wonder why this works. Any ideas? (Students: stunned silence.)

This is what’s known as a cliff-hanger, and I’ll give you the answer at the start of class tomorrow. (Students groan, as they really want to know the answer immediately.) Class is dismissed.

If you’d like to see the answer, see my previous post on this topic.

# Happy Pythagoras Day!

I’d like to wish everyone a Happy Pythagoras Day! Today is 12/20/16 (or 20/12/16 in other parts of the world), and $20^2 = 12^2 + 16^2$.

Bonus points if you can figure out (without Googling) when the next three Pythagoras Days will be.

# Engaging students: Completing the square

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 Deborah Duddy. Her topic, from Algebra: completing the square.

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

Applying what is learned in the class is very vital in fact it is a process TEKS that teachers need to use to maximize student’s understanding. “When are we going to use this in real life?” and “Why do we need to know this?” are questions that students ask on a daily basis. Connecting material to the real world helps engage students and develops critical thinking. Describing a path of a ball, how far an item can be tossed in the air and how to maximize profits for a company are just some examples of how quadratics can be used in the real world.

One important event happens during high school; students receive their driver’s license. In their written driver’s test, students must know the distance needed to stop a car at certain speed limits. Using an example like the one below will be interesting for the students and help connect lesson material and real life.

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

To begin class and get students involved with their learning, the class will participate in an activity. Each pair of students will have two different cards such as (x+2)^2 and x^2+4x+4, and any variations of these problems. They can only look at the (x+2)^2 card. Students will work out the problem on paper. Students will be asked to remember how to find the area of a square and then set up a square with the dimensions matching the first card. From there, the pairs would use algebra tiles (after knowing what each tile stands for) and attempt to “complete the square”. This activity will be used as an engage and a beginning explore for the students. This activity will help students see completing a square geometrically.

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

Completing the square is another way of solving/factoring the equation. The process of completing the square is to turn a basic quadratic   equation of ax^2 + bx + c = 0 into a(x-h)^2 + k = 0 where (h,k) is  the vertex of the parabola. Therefore this process is very beneficial because it helps students graph the quadratic equation given. In order to find h and k, students should be able to factor, square a term, find the square root and manipulate the equation.

In solving the equation by completing the square is to subtract the constant off the left side and onto the right side. Then students take the coefficient off the x-term divide it then square it. Students then add this number to both sides of the equations. By simplifying the right side of the equation, students give the perfect square. Then solve the equation left by taking the square root of both sides and determining x.

References:

http://www.classzone.com/eservices/home/pdf/student/LA205EBD.pdf

# My Mathematical Magic Show: Index

I’m doing something that I should have done a long time ago: collecting a series of posts into one single post. The links below show the mathematical magic show that I’ll perform from time to time.

Part 1: Introduction.

Part 2a, Part 2b, and Part 2c: The 1089 trick.

Part 4a: Part 4b, Part 4c, and Part 4d: A trick using binary numbers.

Part 5a, Part 5b, Part 5c, and Part 5d: A trick using the rule for checking if a number is a multiple of 9.

Part 7: The Fitch-Cheney card trick, which is perhaps the slickest mathematical card trick ever devised.

Part 8a, Part 8b, and Part 8c: A trick using Pascal’s triangle.

Part 9: Mentally computing $n$ given $n^5$ if $10 \le n \le 99$.

Part 6: The Grand Finale.

And, for the sake of completeness, here’s a recent picture of me just before I performed an abbreviated version of this show for UNT’s Preview Day for high school students thinking about enrolling at my university.

# Matlab and Optimal Placement of Christmas Ornaments

It turns out that Matlab can be used to optimally place ornaments on a Christmas tree: https://github.com/tsangiotis/christmasSpirit

# Mathematical Present Wrapping (Part 2)

Yesterday’s post of mathematically wrapping presents was tongue in cheek. This one is elegant and a nice application of principles from geometry.

# Engaging students: Standard Deviation

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 Jillian Greene. Her topic, from statistics: standard deviations.

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

An activity that I’ve seen presented to introduce the idea of standard deviation requires students to explore the information given to them before actually being taught the math behind standard deviation. As the students settle into their seats, prompt them to work with their shoulder partner and help to measure the width of their left thumbnail (or length of their pointer finger, width of their hand, etc.) and write it on a sticky note. Once the data is collected, the students will calculate the mean of all of the measurements. The mean is then written on the board in the center, and the students are asked to go and stack their post-it notes in either the center if they are perfectly the mean, or on the right or left if it’s bigger or smaller, respectively. Have them find the mean of the distances of each measurement from the mean. When they discover this should be zero, have them discuss with each other and then in the big group what that means. If time provides, it might even be fun to ask deeper understanding questions like what would happen if everyone last half of their thumbnail, or what if just Student A’s thumbnail tripled in size. This will provide a meaningful sequitur into the sometimes confusing world of standard deviation and distances from the mean.

How has this topic appeared in pop culture (movies, TV, current music, video games, etc.)?

http://www.dailymotion.com/video/x3lc0rx

This is a full episode of Everybody Loves Raymond but the clip in reference starts at about 8:45 and lasts a minute or so.

This clip shows a scenario where the couple, Ray and Deborah, is comparing their scores on an IQ test (a very common use for standard deviation). Deborah comments on how her score is very close to Ray’s, being only 15 points higher. The brother that proctored the exam corrected her by saying that 15 points is a standard deviation higher and puts her in a “whole new class” of genius. Have students discuss and explain what it means for Deborah to be one standard deviation higher. Use the information given in the episode (100 is average, 115 is one standard deviation higher) to construct the bell curve for IQ scores. Then use the bell curve to introduce percentiles. Since Ray is the average, center-of-the-bell score, then he is in the 50th percentile. The students can then attempt to discover on their own (or with a group) what percentile Deborah’s score puts her in.

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

Standard deviation is a topic that pervades almost all sciences. In biology classes, students are asked to student the weather and climate of various habitats. In differentiating between the two, one must look at the overall picture. If the student is presented with the information that place A and place B both have average temperatures of 60 degrees, this information might not be good to take as face value. Place A might have a range from 40 to 80 degrees throughout the year while place B might range from 0 to 100 and then have one or two extremely hot outliers that even the average out to 60. Looking at not only the skew of the bell curve, but also what the standard deviation is for each place, might save a student from forgetting to bring a fan to hypothetical place B, or writing that that the climate of that place is cool year round.  In addition to biology, standard deviation is a very necessary operation in psychology, which is a very statistics-based science. This can easily be seen in representing IQ scores how we found earlier!

# Engaging students: Introducing proportions

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 Deborah Duddy. Her topic, from Pre-Algebra: introducing proportions.

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

Proportions, in the form a/b = c/d, is a middle school math topic. The introduction of proportions builds upon the students’ understanding of fractions and ability to solve simple equations. This topic is used in the students’ future Geometry and Statistics courses. The use of proportions is used in Geometry to identify similar polygons which are defined as having congruent corresponding angles and proportional corresponding sides. The use of similar triangles and proportions are used to perform indirect measurements. In Statistics, proportions are used throughout measures of central tendency. Additionally, statistics uses sampling proportions including the proportion of successes.

The ability to use proportions for indirect measurements is also included in the study of Physics, Chemistry and Biology.  Chemistry uses proportions to determine based upon the chemical structure of a compound, the number of atoms pertaining to each element of the compound.  The study of Anatomy also uses many proportions including leg length/stature or the sitting height ratio (sitting heigh/stature x 100).

How has this topic appeared in high culture (art, classical music, theatre, etc.)?

In art, proportions are expressed in terms of scale and proportion.  Scale is the proportion of 2 different size objects and proportion is the relative size of parts within the whole.  An example of proportion is Michelangelo’s David.  The proportions within the body are based on an ancient Greek mathematical system which is meant to define perfection in the human body.  Da Vinci’s  Vitruvian Man is also an example of art based upon proportions or constant rates of fractal expansion.  The music of Debussy has been studied to show that several piano pieces are built precisely and intricately around proportions and the two ratios of Golden Section and bisection so that the music is organized in various geometrical patterns which contribute substantially to its expansive and dramatic impact.

The use of proportions is also a constant within Greek and Roman classical architecture.  Many classical architecture buildings such as the Parthenon illustrate the use of proportions through the building.  Additionally, classical architecture uses specific proportions to determine roof height and length plus the placement of columns.

How has this topic appeared in the news?

Proportions are constantly in the news even though they may not be presented in a/b=c/d format.  However, the concept of proportion is used throughout news reporting and even advertising.  The current news topic is the upcoming Presidential election.  Daily, we are provided with new and different poll results.  These results are derived via a proportion.  For example, 100 people are polled, these results are then derived via proportional concepts to provide a percentage voting for each candidate.  Percentage is a specific type of the  a/b = c/d proportion.  Daily news uses proportions when reporting growth trends for national debt, crime and even new housing starts in DFW.   Today, proportions were used when discussing the new Samsung Note7 and its ability to explode.  During the winter, proportions are used to tell us how many inches of rain would result from 2 inches of snow. Sports broadcasters also use proportions when discussing the potential of athletes.  If the athlete can hit 10 homeruns in 20 games, then he will potentially hit 50 homeruns in 100 games.  Proportions even appear in advertising for new medicines detailing the data associated with the medicine trial.

References:

Debussy in Proportion: A Musical Analysis, Dr Roy Howat

Michelangelo’s David

https://abcnews.go.com/images/PollingUnit/MOEFranklin.pdf

http://www.brightstorm.com