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Circles With and Without Coordinates

Modeling With Circles

In the previous lessons, all the features and theorems of the circles have been covered. This lesson will focus on real-life examples that can be modeled by circles.

Catch-Up and Review

Here are some recommended readings before getting started with this lesson.


Investigating Circular Shaped Objects

Tennis balls are sold in cylindrical containers that contain three balls each. The tennis balls and container can be modeled as follows.

Tennis Balls

The container, lying on its side, passes through an X-ray scanner in Tallahassee International Airport, where Jordan works as a security guard. Depending on the opacity of the container and the balls, what should Jordan expect to see when she reads the screen of the X-ray scanner? Explore the applet by adjusting the opacity of the container and the tennis balls.

X-ray Scanner


Modeling the Area of Circular Objects

Izabella and her friends are ordering pizza from their favorite spot. They can afford to buy either two 9-inch pizzas or one 13-inch pizza. The thickness of the pizza is the same for each.


They cannot quite decide which option means more food. By modeling each pizza as a circle, help Izabella and her friends determine the best option.


Since both pizzas have the same thickness, the area of the pizzas will help to make a decision.


It is given that the pizzas have the same thickness. Therefore, areas of the pizzas will determine the best option. Begin by modeling the pizzas as circles with diameters of 9 and 13 inches.

Because the radius of a circle is half of its diameter, the radii of the pizzas can be found by dividing their diameter by 2.
Now that the radii of the pizzas were found, the formula for the area of a circle can be used to find their areas. Begin by finding the area of 9-inch pizza.
Evaluate right-hand side
The area of the pizza is about 63.62 square inches. Following the same procedure, the area of 13-inch pizza can also be found.
Radius Substitution Area
9-inch pizza 4.5 A=π(4.5)2
13-inch pizza 6.5 A=π(6.5)2
Recall that Izabella and her friends can afford two 9-inch pizzas.
As a result, the best option is to buy a 13-inch pizza. One of the friends was so close to ordering the two 9-inch pizzas. Thanks for helping them make the right choice.


Modeling the Radius of Circular Objects

The Knights of the Round Table are the highly-esteemed knights of the mythical fellowship of King Arthur. In the beginning, there were only twelve knights. They met at the Round Table that is a symbol of equality among all of its legendary members.

The Knights of Round Table

Assume that each knight has the same circular chair around the Round Table whose radius is 10 feet. Given that all chairs and the Round Table are externally tangent to each other, what is the radius of each chair? Round the answer to the nearest tenth.


Begin by modeling the Round Table and the chairs using circles. Draw a triangle whose vertices are the center of the inner circle and two adjacent outer circles. Then use the trigonometric ratios.


The Round Table and the chairs can be modeled by using circles.

Twelve Circles

By labeling the centers of the inner circle and two adjacent outer circles, a triangle passing through these centers can be drawn.

Twelve Circles

Segments that connect the centers of the tangent circles pass through the points of tangency. Assume that the radius of each chair is r feet.

Twelve Circles
Notice that ABC is an isosceles triangle where AB=AC. From here, to find the value of r, the vertex angle of ABC needs to be found. Since there are 12 chairs, the measure of BAC can be found by dividing by 12.
Now that the measure of the vertex angle was found, the altitude of the base can be drawn to use the trigonometric ratios. Remember that the altitude of the base of an isosceles triangle bisects the vertex angle and the base.
Twelve Circles
Finally, using the sine ratio in ABD, the value of r can be found.
Solve for r
Therefore, the radius of each chair is about 3.5 feet. For reference, most common chairs these days are about 16 inches in diameter.


Modeling Bodies Using Cylinders

Jordan and Ali are comparing their weights. Jordan is 5 feet tall, and Ali is 6 feet tall. Their shoulder widths are 1 foot and 1.4 feet, respectively.

Modeling Human Bodies With Cylinders

Consider their bodies to be roughly cylindrical, and their shoulder widths could represent the diameter. What is the ratio of Jordan's weight to Ali's weight?


4:9 or


Volume of a cylinder is the product of the base area and height of the cylinder.


When a body is modeled using a cylinder, one's height and shoulder width will represent the cylinder's height and diameter, respectively.


By comparing the volumes of these cylinders, a relation between the weights of Jordan and Ali can be set up. To do this, the radii of the cylinders should be found. Recall that the radius of a cylinder is half of its diameter.

Now that the radii have been found, the formula for the volume of a cylinder can be used to find their volumes.

h r πr2h V
Shorter Cylinder 5 0.5 π(0.5)2(5)
Taller Cylinder 6 0.7 π(0.7)2(6)

The volumes of the cylinders imply that if Jordan weighs 80 pounds, Ali weighs 180 pounds. Therefore, the ratio of Jordan's weight to Ali's weight is 4:9 or


Modeling Arc Measures

Arc measure is also an important concept to interpret the situations modeled by circles. For example, consider a time zone wheel.

A time zone wheel is a tool used to find the time in different locations across the world. For example, to find the time in Tokyo when it is in Amsterdam, rotate the small wheel until the labels of and Amsterdam align.


As can be seen, it is in Tokyo.

a What is the arc measure between each time zone on the wheel?
b What is the measure of the minor arc from the Istanbul zone to the Los Angeles zone?


a How many time zones are there on the wheel?
b How many time zones are there between the Istanbul zone and Los Angeles zone?


a It can be seen that the wheel is divided into 24 time zones. Since the measure of the whole wheel is the arc measure between each time zone on the wheel can be found by dividing by 24.
b Notice that the minor arc between the Istanbul zone and Los Angeles zone contains 11 time zones. Since the arc measure between each time zone is the measure of the minor arc can be found by multiplying by 11.


Modeling Real Life Situations Using Circles

In real life, there are plenty of situations where circles can be appreciated. In this lesson, a few common situations have been covered. How about some other fun ones? Take, for example, one of the most complex applications of circles — the global positioning system.

When the three circular regions scanned by each satellite intersect, the position of the device is determined. Much more detail can be learned about GPS, this is just a brief introduction. How else are circles used in everyday life?
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