## Isosceles Triangle Theorem Lesson

### What is the Isosceles Triangle Theorem?

The isosceles triangle theorem tells us that:

**If two sides of a triangle are congruent, then the angles opposite those sides are congruent.**

The angle opposite a side is the one angle that does not touch that side. See the image below for an illustration of the theorem.

INTRODUCING

### When do we use the Theorem?

This theorem is useful when solving triangle problems with unknown side lengths or angle measurements. The theorem goes both ways, meaning we can:

- Assume two angles are congruent from knowing two sides are congruent.
- Assume two sides are congruent from knowing two angles are congruent.