4-5

Isosceles and Equilateral Triangles

Learning Goal

1. To use and apply properties of isosceles and equilateral

triangles.

Vertex angle: The angle formed by the two congruent sides of an

isosceles triangle.

Base: Side opposite the vertex angle of an isosceles triangle.

Legs: Congruent sides of an isosceles triangle.

Parts of an Isosceles Triangle

Draw this big and label it with the definitions.

Vertexangle

Base

Leg Leg

Baseangle

Baseangle

Isosceles Triangle Theorem

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

congruent.

If ON = OJ, then J = N

O

J

N

~ ~

Converse of Isosceles Triangle Theorem

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

congruent.

If S = A, then SM = AM

S

A

M

~ ~

Theorem 4-5

If a line bisects the vertex angle of an isosceles triangle, then the

line is also the perpendicular bisector of the base.

Theorem 4-5

If a line bisects the vertex angle of an isosceles triangle, then the

line is also the perpendicular bisector of the base.

Turn to page 253.

10

1-3, 11-17, 25, 28, 30-32

5 Points: 100% Complete 4 Points: 80% Complete 3 Points: 60% Complete 2 Points: 40% Complete

1 Point: 20% Complete