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Learning Math Home
Geometry Session 4, Part C: Circles
Session 4 Part A Part B Part C Part D Homework
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Session 4 Materials:

Session 4, Part C:
Circles (35 mintues)

In This Part: Inscribed Angles | More Circle Constructions

A circle is the set of all points in a plane that are equidistant from a given point in the plane, called the center of the circle. Note 4

Problem C1


Construct at least three circles of different sizes. For each circle, complete steps (a)-(e).


Construct the circle's diameter. (A circle's diameter is the segment that passes through the center and has its endpoints on the circle.) Constructing the diameter of a circle creates two semicircles.

Stop!  Do the above problem before you proceed.  Use the tip text to help you solve the problem if you get stuck.
To construct the diameter of a circle, you can't simply draw a segment that looks like it goes through the center; you need to make sure it's connected to the center of the circle. Here's one way: Change the Segment Tool to a Ray Tool (or Line Tool) by clicking and holding on that button until the other options appear; then select the one you want. Draw a ray or line with one point on the circle and another point connected to the circle's center. (Watch your cursor carefully to make sure it doesn't just look right, but that it really connects to the center.) If you wish, you can draw a segment between the two points where the ray intersects the circle, then hide the ray.   Close Tip



Construct an inscribed angle in one of the semicircles. (An angle is inscribed in a circle if its vertex is on the circle and its rays intersect the circle. For an angle to be inscribed in a semicircle, the rays must intersect the circle at the endpoints of a diameter.)


Measure your inscribed angle angleVXW.


Grab the vertex of your inscribed angle with the Pointer Tool and move it around the circle. How does this affect the measure of the angle?


What conjecture can you make about the measure of an angle that is inscribed in a semicircle?


The following Interactive Activity allows you to investigate the inscribed angles in a semicircle. Does the activity corroborate what you found in problem C1 (d)? Make a conjecture about the inscribed angle in a quarter-circle and use the activity to corroborate this conjecture as well. Please note that due to technical limitations, this activity will not draw angles lower than 3°. Theoretically, you could go down to 0° and get a straight line.

This activity requires the Flash plug-in, which you can download for free from Macromedia's Web site. If you prefer, you can view the non-interactive version of this activity, which doesn't require the Flash plug-in.


Problem C2


What conjecture can you make about the measure of an angle that is inscribed in a quarter-circle?

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Video Segment
In this video segment, Ric and Michele construct an inscribed angle in a semicircle, then measure the angle as they move it around the circle. Watch this segment after you have completed Problem C1 and compare your results with those of the onscreen participants.

What did Michele and Ric discover about their inscribed angle as they moved it around the semicircle? How does this compare with your findings?

If you are using a VCR, you can find this segment on the session video approximately 10 minutes and 50 seconds after the Annenberg Media logo.


Next > Part C (Continued): More Circle Constructions

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