# Geometry/Chapter 4/Lesson 4

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## Introduction[edit]

This lesson will go through Isoscolese Triangles and its principles.

## Isocsolese Triangles[edit]

**Isoscolese Triangles**, as mentioned in Lesson 2, is a triangle that has **at least 2 sides that are congruent**. The congruent sides of an isoscolese triangle are the **legs**. The third side is the **base**. The two *legs* form the **vertex angle**. The other two angles are the **base angles**.

There are quite a few **theroems** that accommodate one's mind to understanding Isosceles Triangles, such as Theorems **4-1, 4-2**, and **4-3**:

### Theorem 4-1[edit]

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

### Theorem 4-2[edit]

- The bisector of the vertex angle of an isosceles triangle is the perpendicular bisector of the base.

### Theorem 4-3[edit]

- If two angles of a triangle are congruent, then the sides opposite the angles are congruent. (Basically, Theorem 4-1 reversed)