# Circumscribed circles

The circumscribed circle is the circle passing through the three vertices of a triangle ABC. Assume the following results from geometry. The perpendicular bisectors of the sides of a triangle meet in a point O that is the center of the circumscribed circle.

a) According to a theorem from geometry, the measure of the angle AOC is twice that of angle B. What theorem is this? (State the theorem in complete sentences)

b) Draw a perpendicular from the center O to AC, meeting AC at T. Denoting the length of the side opposite to the angle B by b, explain why AT=TC-b/2.

c) Explain why triangle ATO is congruent to triangle CTO.

d) Use the results in parts (a) and (c) to show that angle COT = angle B.

e) Us the result in part (d) to show that the radius R of the circumscribed circle for triangle ABC is equal to b/(2 sin B). The use the law of sines to conclude from this that

R = ___a_____ = _____b______ = _____c_____

2 sin A 2 sin B 2 sin C

f) Let A denote the area of the triangle ABC. Show the radius R of the circumscribed circle is given by

R = __abc___

4A

hint: use the result from part (e)

#### Solution Preview

Please see the attachment.

The above graph is for this problem.

a. The theorem is: the angle at the center is twice of the angle of circumference for a given chord in a cicle.

b. In your problem, the side opposite to the angle is . So . Since is ...

#### Solution Summary

This uses a circle circumscribed through the vertices of a triangle, and answers questions regarding congruency and angle measures.