1. Objectives Prove theorems about isosceles and equilateral triangles.
Apply properties of isosceles and equilateral triangles.

2. Vocabulary
legs of an isosceles triangle
vertex angle
base
base angles

3. Recall that an isosceles triangle has at least two congruent sides.
_____ - The congruent sides of an isosceles triangle.
____________– The angle formed by the legs.
______ – The side opposite the vertex angle.
______________– The two angles that have the base as a side.
 is the vertex angle.
 and  are the base angles.

5. The Isosceles Triangle Theorem is sometimes stated as “Base angles of an isosceles triangle are congruent.”
Reading Math

6. Example 1A: Astronomy Application
The length of YX is 20 feet.
Explain why the length of YZ is the same.

7. Example 2A: Finding the Measure of an Angle
Find mF.

8. Example 2B: Finding the Measure of an Angle
Find mG.

10. Example 3A: Using Properties of Equilateral Triangles
Find the value of x.

11. Example 3B: Using Properties of Equilateral Triangles
Find the value of y.