## All posts tagged **circle**

# Thanksgiving warning

*Posted by John Quintanilla on November 17, 2017*

https://meangreenmath.com/2017/11/17/thanksgiving-warning/

# Engaging students: Finding the equation of a circle

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 Lucy Grimmett. Her topic, from Precalculus: finding the equation of a circle.

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

I love doing activities to teach topics, or solidify students knowledge after direct teaches. The link below describes an activity/project a teacher did with her students. The students were asked to create an image using circles. They can use other images along with circles; however, circles are the main focus of the project. The picture had to include at least four circles. Once they had drawn their image containing the circles they were asked to find the equations of each of the circles in their picture. As an extra challenge students were asked to create a question to go with one of their circles that would aid another student in finding the equation. This is where students have to truly put two-and-two together to create an in depth connection between the lesson, word problems, and furthermore the idea of the center of a circle and the length of the radius.

http://secondarymissrudolph.blogspot.com/2012/04/equations-of-circles-update.html

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

In Algebra II students learn about writing equations of graphs and how each piece of the equations manipulates the graph. This is a skill that continues into Pre-Calculus. Whether students are graphing circles, exponential functions, or trigonometric equations there is always variables that can be manipulated that manipulate the graph. Further, students will be required to find equations of hyperbolas, ellipses, and parabolas. These equations go hand and hand with one another when students are using a focus and directrix. As you know, mathematics constantly builds on itself. With students previous knowledge of quadratic equations specifically, they see how in the equation , is the “center” of the graph. The same goes for the equation of a circle. The point is the literal center of the circle in the formula . With their previous knowledge of h affecting the x component and k affecting the y, students are able to grasp the concept more quickly, and more efficiently.

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

In Algebra II students learn about writing equations of graphs and how each piece of the equations manipulates the graph. This is a skill that continues into Pre-Calculus. Whether students are graphing circles, exponential functions, or trigonometric equations there is always variables that can be manipulated that manipulate the graph. Further, students will be required to find equations of hyperbolas, ellipses, and parabolas. These equations go hand and hand with one another when students are using a focus and directrix. As you know, mathematics constantly builds on itself. With students previous knowledge of quadratic equations specifically, they see how in the equation , is the “center” of the graph. The same goes for the equation of a circle. The point is the literal center of the circle in the formula . With their previous knowledge of h affecting the x component and k affecting the y, students are able to grasp the concept more quickly, and more efficiently.

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

There are so many online resources to use for mathematics teaching. Students have easy access to an online graphing calculator called Desmos. This allows students to play with different numbers in equations to see how they affect the graph. For examples, students can manipulate the radius and the center point. This will allow students to visually see how each variable contributes to the equation. Below are other links that are beneficial when teaching equations of circles. Khan Academy is a tool that is used by many educators, not only does Khan Academy include instructional videos, but it also has mini quizzes after that check for student knowledge. The second link below is an online resource which student can insert equations of circles and the program will give them the radius, center, a graph, and it will even give an explanation. There is a second mode on this resource which student put the radius and center and the site will return the equation, again, the website will give an explanation if needed. These resources are quick ways for student to see how the equation of a circle can change different pieces of circles.

http://www.mathportal.org/calculators/analytic-geometry/circle-equation-calculator.php

*Posted by John Quintanilla on July 14, 2017*

https://meangreenmath.com/2017/07/14/engaging-students-finding-the-equation-of-a-circle-3/

# Engaging students: Finding the area of a circle

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 Daniel Herfeldt. His topic, from Geometry: finding the area of a circle.

Learning area of a circle as well as the circumference in geometry is beneficial for anyone who is looking to pursue any career involving math. This includes anything from a math teacher to an architect. This helps with most future courses in mathematics. With this said, it will be very beneficial when going into pre-calculus. This is because in pre-calculus you will deal a lot with trigonometry. This includes such things as the unit circle, which is a great deal to pre-calculus itself. Being familiar with the equations of a circle helps to understand why things work in a unit circle. It can help with simple things such as why x^{2}+y^{2}=r^{2}. Knowing the area of a circle will make the class easier to understand in all. This topic is also very important to future architects. The reason for this is because if an architect doesn’t know the area of a circle or any other shape, it would be very difficult to construct a building. If one cannot figure the dimensions of a pillar to help support the ceiling of a building, the building will have a possibility to collapse. This causes the structure of a building to rely highly on the dimensions, area, and volume of all shapes including the circle. This proves that the importance of the area of a circle to be very high. Most students will not know that everywhere they go, circles are needed. Informing them about these small details could have the students more eager to learn. Giving them great real world examples might also help the students understand and grasp the knowledge that you are trying to teach them because it relates to them.

Circles will be anywhere you go. They are in your everyday TV show, video games, and movies. Although at first glance you might not actually see them, they really are there. When creating a character for an animated movie or a popular video game, artist first start to draw with simple circles and lines. They need to figure out a certain area of the character’s face to be able to fit the facial features. For example, they need to be able to fit eyes, a mouth, a nose and a few more features. From this, they will go on to the animation of the character. This also includes circles because in an animation, when you are wanting to move one object, you have to move it all. The same process applies when working on the landscape properties. It will mostly start with simple lines, circles, and boxes. From there, it will progress into more advance steps, putting more and more detail into it. When moving a character, it is also necessary to move the landscape and surroundings as well. This would be great to tell a class because students will be able to relate to the subject. Most kids in the high school level will play video games. Whether the game is on their phone or a gaming console, they still require the beginning steps. If the student doesn’t play video games, they can relate to it due to watching an animated movie. This will be a great way to engage the class in the first few minutes of class. Below is a picture of the progression of drawing a Pokémon.

Many ancient civilizations have been fascinated with circles. Circles can be seen in many ancient structures and buildings from the Roman Coliseum to Stonehenge.

One ancient civilization fascinated with circles were the Greeks. The believed the circle to be a sacred divine shape mostly based on its multiple points of symmetry. The Greeks also invented a puzzle called squaring the circle, in which the person had to construct a square with the exact area of a circle a compass and a straight edge. This puzzle has been proved mathematically impossible.

Other instances in which circles played an important role in history were the circles that appeared in the crops in different areas of the world. These crop circles have been argued to be a hoax while others indicate it is not possible for the crop circles to be the work of humans. Regardless, of their origin, these crop circles continue to fascinate us.

Circles continue to have significance today. They are used in logos and other things usually to signify unity and harmony. Even the Olympic symbol is made up of five interlocking colorful rings. The circle is still found today enclosing the all seeing eye over the pyramid in the dollar bill on the US currency.

The significance of presenting this information to students, especially high school students, is to give them background information. I think many high schoolers will be interested to learn how circles have been significant in other cultures throughout history. Students can be given a short introduction in the subject and asked to look for more instances in which circles played a role in an ancient civilization and then bring that trajectory to modern times.

Website: https://nrich.maths.org/2561

*Posted by John Quintanilla on May 30, 2017*

https://meangreenmath.com/2017/05/30/engaging-students-finding-the-area-of-a-circle-4/

# 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,

,

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

- If , a constant, then .
- If and are both differentiable, then .
- If is differentiable and is a constant, then .
- If , where is a nonnegative integer, then .
- If is a polynomial, then .

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 . Notice I’ve changed the variable from to , but that’s OK. Does this remind you of anything? (Students answer: the area of a circle.)What’s the derivative? Remember, is just a constant. So .

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 . Does this remind you of anything? (Students answer: the volume of a sphere.)What’s the derivative? Again, is just a constant. So .

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.

*Posted by John Quintanilla on April 29, 2017*

https://meangreenmath.com/2017/04/29/my-favorite-one-liners-part-88/

# Math Maps The Island of Utopia

Under the category of “Somebody Had To Figure It Out,” Dr. Andrew Simoson of King University (Bristol, Tennessee) used calculus to determine the shape of the island of Utopia in the 500-year-old book by Sir Thomas More based on the description of island given in the book’s introduction.

News article: https://www.insidescience.org/news/math-maps-island-thomas-mores-utopia

Paper by Dr. Simoson: http://archive.bridgesmathart.org/2016/bridges2016-65.html

*Posted by John Quintanilla on September 30, 2016*

https://meangreenmath.com/2016/09/30/math-maps-the-island-of-utopia/

# Look at my tan line

*Posted by John Quintanilla on September 26, 2016*

https://meangreenmath.com/2016/09/26/look-at-my-tan-line/

# A natural function with discontinuities (Part 3)

This post concludes this series about a curious function:

In the previous post, I derived three of the four parts of this function. Today, I’ll consider the last part ().

The circle that encloses the grey region must have the points and on its circumference; the distance between these points will be , where is the radius of the enclosing circle. Unlike the case of , we no longer have to worry about the origin, which will be safely inside the enclosing circle.

Furthermore, this line segment will be perpendicular to the angle bisector (the dashed line above), and the center of the enclosing circle must be on the angle bisector. Using trigonometry,

,

or

.

We see from this derivation the unfortunate typo in the above *Monthly* article.

*Posted by John Quintanilla on August 22, 2016*

https://meangreenmath.com/2016/08/22/a-natural-function-with-discontinuities-part-3/

# A natural function with discontinuities (Part 2)

Yesterday, I began a short series motivated by the following article from the *American Mathematical Monthly*.

Today, I’d like to talk about the how this function was obtained.

If , then clearly . The original circle of radius clearly works. Furthermore, any circle that inscribes the grey circular region (centered at the origin) must include the points and , and the distance between these two points is . Therefore, the diameter of any circle that works must be at least , so a smaller circle can’t work.

The other extreme is also easy: if , then the “circular region” is really just a single point.

Let’s now take a look at the case . The smallest circle that encloses the grey region must have the points , , and on its circumference, and so the center of the circle will be equidistant from these three points.

The center must be on the angle bisector (the dashed line depicted in the figure) since the bisector is the locus of points equidistant from and . Therefore, we must find the point on the bisector that is equidistant from and . This point forms an isosceles triangle, and so the distance can be found using trigonometry:

,

or

.

This logic works up until , when the isosceles triangle will be a 45-45-90 triangle. However, when , a different picture will be needed. I’ll consider this in tomorrow’s post.

*Posted by John Quintanilla on August 21, 2016*

https://meangreenmath.com/2016/08/21/a-natural-function-with-discontinuities-part-2/

# Engaging students: Radius, Diameter, and Circumference of a Circle

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 Zacquiri Rutledge. His topic, from Geometry: radius, diameter, and circumference of a circle.

There are many ideas about how to introduce students and have them study the relationships between the radius, diameter and circumference of a circle. However, one of my favorites has always been the month long project assigned to students at the beginning of class. On the very first day of class, the teacher is to assign the students their project. The instructions of this project are for each of the students to find and measure ten different round or circular objects around their home. The students will need to measure the length around the object (the circumference) using a piece of string and a ruler (the teacher might explain to the students or give an example so they know how to do this), the length from one side of the object to the other side passing through the middle (diameter), and the length from the center of the object to the outside (radius). If the students already know what these terms are called that is okay. However, the teacher should avoid explaining these terms until later.

Then a month later, the students are to bring their findings to class. At this point during the class the teacher will have begun her segment of lessons about circles and the various properties of circles. By now the students should have a good idea what the terms radius, diameter, and circumference mean. So the day the students bring in their work, they will be given the following chart, originally designed by the University of Illinois. From here students will slowly begin to fill in their charts with the information they gathered. Once completed students will then begin finding the ratios between diameter-radius and circumference-diameter and recording them. Finally at the bottom, students will find the average of their ratios from the last two columns. Once all of this data is completed, the students should have found that the diameter and radius share a ratio of 2-1 since the diameter is twice the radius. The last column should have produced something close to an average of 3.14159265359 or better known as pi (). Not only will this help students understand that pi is not just a number, but it will also help them to know where it comes from and its importance. From here the teacher would be able to lead into a lesson about some of the other uses of pi and how they all relate back to the relationships between radius, diameter and circumference.

Radius, diameter, and circumference are very important in many topics beyond their definitions. For instance, later on in the geometry course students will talk about the area of circles. Even though the students might have learned how to find the area of simple polygons such as triangles and quadrilaterals, finding the area of a circle is different because of the use of pi. To find the area of a circle, students have to recall the relationships between the radius, diameter, and circumference in order to understand how the area of a circle uses those relationships. Another example of how they are used is in pre-calculus. In pre-calculus students will talk about the unit circle, a circle with a fixed radius of 1 unit. Using the fixed radius of 1, students will discover that the length around (circumference) the unit circle is 2π. This 2π is important because it can be broken into pieces, called radians, and used to help measure Sine, Cosine, and Tangent at certain radians around the circle. Learning about sine, cosine, and tangent opens up even more things for the students, such as trigonometry and calculus. However, no matter how advanced the mathematics become, they always relate back to the simple concepts of the radius, diameter and circumference of a circle and their relationships.

Radius, diameter, and circumference is a topic that has been talked about and used dating back to 2000 B.C. But, what has it actually been used for all this time? How about architecture? Think about massive constructs such as the Theatre of Ephesus in Rome, Italy. Even though the theatre is not a full circle, look at how each of the seats are evenly placed from the stage. This is because when it was designed, the architect likely used the radius and circumference to accurately plot how far each seat needed to be placed in order to be the same exact distance from the stage as everyone else in their row. Even though only half a circle was used for this theatre, the circumference and radius would have been used to find the ratio pi in order to get the area of how much space was allowed for seating.

Another great example of circumference being used is in the invention of the clock. The clock originated as a sun dial, which would use the sun to cast a shadow, which would tell the time of day. These sun dials date back as early as 3500 B.C. However, in 1583 Galileo found a way to use a pendulum to create a clock that always followed the same length of time (Clock). This is important because not long after the first clock was born, so was the circular face of a clock. The face of a clock has the numbers 1-12 on it, each one evenly spaced around the edge of the clock. By using the circumference of any size of circle, the person building the clock would know just how far to space out each of the numbers, giving each hour the same amount of time between them. If even one of the numbers were off on the clock, the time would be off. Also, it can be seen that on modern clocks, the minute hand always stretches the radius of the clock. By stretching out the minute hand on the clock, the designer of the clock can create evenly spaced notches on the face using the circumference, in order to have the minute hand indicate the minute of the hour.

One final example is the use of radius in war, or more specifically the invention of the radar. Radar was originally being experimented with by German physicist Heinrich Hertz in 1887. He had discovered that certain materials allowed radio waves to pass through them, while others reflected them. In 1890, Nikola Tesla realized that large objects could reflect large enough radio waves to be detected. By harnessing this idea, pulse radar would come to be introduced into United States in 1925, and later used in the British Air Force to defend against German air raids during WWII (Science). The reason radar works, however, is because the system has a set radius in which it can detect radio waves. Once the radar system sends out a radio wave, if it does not reflect back within the radius of the detection system, then the radar will not pick up on anything. The system measures the distance by measuring how long it takes for the radio wave to return to the system after it is sent out and comparing that time to radius of detection. This allowed not only military to defend against air attacks, but it was commonly used during naval combat to defend against submarines as Germany used their U-Boats to attack several American and British naval ships during WWII, as well as WWI before the invention of radar.

References:

“Circumference and Pi.” *Circumference and Pi*. N.p., n.d. Web. 08 Oct. 2015.

“Clock a History – Timekeepers.” *Clock a History – Timekeepers*. N.p., n.d. Web. 08 Oct. 2015.

“Science Explorations: Journey Into Space: Radar and Sonar | Scholastic.com.” *Science Explorations: Journey Into Space: Radar and Sonar | Scholastic.com*. N.p., n.d. Web. 08 Oct. 2015.

*Posted by John Quintanilla on June 2, 2016*

https://meangreenmath.com/2016/06/02/engaging-students-radius-diameter-and-circumference-of-a-circle/

# Engaging students: Finding the area of a circle

*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.

This student submission comes from my former student Joe Wood. His topic, from Geometry: finding the area of a circle.

The students would be greeted with Lion King’s “Circle of Life” song. While the song has nothing to do with area of a circle, it would create a different and exciting buzz in the classroom that wouldn’t always be offered in this form. (Plus, who doesn’t want to hear a little Lion King music?)

**C1**. How has this topic appeared in pop culture (movies, TV, current music, video games, etc.)?

**A1**. What interesting (i.e., uncontrived) word problems using this topic can your students do now?

A great activity I found on the Mathbits website tackles both questions C1 and A1.

The two-page worksheet below is based off a scene from the movie Castaway. In the scene, Tom Hanks calculates the area of a circle to figure out the likelihood of his rescue. He then compares his calculated area to the area of Texas (which for young students who are all about Texas like I was, this is another attention getter on its own). I would show the clip (having sent a permission slip home since Tom Hanks is shirtless) which can be seen at https://www.youtube.com/watch?v=y89VE9_2Cig so that students can have a good laugh and also understand the scene described on the worksheet. While most, if not all students will never be stranded on a deserted island, this would be an interesting real world problem for the “survivalist” kid in the class.

The worksheet is great because it starts off asking if Tom’s calculations were even correct. It then has several example problems for area of a circle so they can practice, but it also brings in linear speed calculations, and a circumference problem which is great review (and a good warm up if you were maybe moving into angular speed later).

**A1**. What interesting (i.e., uncontrived) word problems using this topic can your students do now?

Another interesting real world problem can be found at http://spacemath.gsfc.nasa.gov/geometry.html. The problem deals with solar energy on satellites (or solar panels in general). It talks about how much energy is needed to operate a satellite, tells the student how much energy is provided by solar cells per square centimeter, gives them different shaped solar panels, and ask is the solar panel can produce enough energy.

This specific worksheet only uses half of a circle on one problem, so it should be revised by the teacher to include more circles; however, once again, I think keeping all the different shapes is a great review for students. I also think having the semicircular shaped panel is a great idea to keep the students on their toes.

**A2. **How could you as a teacher create an activity or project that involves your topic?

If one stereotype can be made about middle/high school students (especially the boys), it is that they love to eat! And, what do they like to eat? PIZZA! There are several ways this next idea could be carried out (pun intended), but for the purposes of this assignment I will call it a class project that ends in a pizza party.

The idea is that each pair of student will be assigned a pizza restaurant in the area, and they will do a presentation on why we should order pizza from this pizza place specifically. They will have find all the pizza sizes (small, medium, large, etc.) , their prices, their diameters, the areas of each pizza, the price per square inch of each of the pizzas, and the best buy. They can talk about anything else they want (such as quality vs price or customer service or whatever) so long as they are trying to sway the class on why the pizza should be purchased from this specific place. Finally, the students will need to provide some kind of proof of their work (menus, calculations, etc) in an organized fashion: PowerPoint, poster board, or some other method.

After the project is complete, the teacher can select the place to buy from, or hold it to a class vote, and have a pizza party during lunch hour or after school or in class.

*Posted by John Quintanilla on May 25, 2016*

https://meangreenmath.com/2016/05/25/engaging-students-finding-the-area-of-a-circle-3/