Page 226 - ABCTE Study Guide_Neat

Example of exterior angle

Question

Assuming that ACD is an equilateral triangle, what is the measure of
ACB?

A 60°

B 90°

C 120°

D 135°

Answer

The correct answer is C. Since ACD is an equilateral triangle, it is also equiangular. Thus, each of its
angles measures 60° and m ACD = 60°.
BCD is a straight angle, so it measures 180°.
According to the figure, (this relationship is actually referred to as the angle
addition postulate). Now substitute:

What can be learned from this one example? Well, this relationship is true for all polygons: The sum of the
measures of a pair of interior and exterior angles of a polygon is 180°. That is, they are supplementary.