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# 11.1: Diameter and Circumference of a Circle

Difficulty Level: At Grade Created by: CK-12

This activity is intended to supplement Geometry, Chapter 10, Lesson 4.

In this activity you will

• Draw a circle
• Measure the diameter of the circle
• Measure the circumference of the circle
• Calculate the ratio of the circumference to the diameter.

1. Open Cabri Jr. and select New. Construct a circle.
2. Draw a line through the two points which determined the circle.
3. The line intersects the circle twice, but only one of these intersection points is marked. Mark the other intersection point by choosing Point, and then Intersection, from the F2\begin{align*}F2\end{align*} menu. The point is ready to be marked when both the circle and the line are flashing. Now we have two points on the circle which are the endpoints of a diameter.
4. Choose Measure and then D. & Length from the F5\begin{align*}F5\end{align*} menu. Find the length of the diameter by selecting each of its endpoints. Show the measurement rounded to the hundredths and move it to a convenient location.
5. With the Measurement tool still active, find the circumference of the circle. Show the measurement rounded to the hundredths and move it to a convenient location.
6. Choose Calculate from the F5\begin{align*}F5\end{align*} menu. Select the circumference measurement, press the division key, and then select the diameter measurement. The number displayed is the ratio of the circle’s circumference to its diameter.
7. Turn off the Calculate tool and grab the center of the circle. Move it to change the size of the circle.
8. To confirm that the ratio remains 3.14, repeat the Calculate procedure. (It is actually recalculated each time the size of the circle changes, but it is impossible to tell since this number is unchanging.)

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