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# Isosceles Triangle Theorem

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