Lesson 4-5: Isosceles & Equilateral Triangles

VocabLegs

Base Angles

Equiangular

Bisects

The congruent sides of an isosceles ∆

Congruent angles opposite the legs of an isosceles ∆

A triangle with all three angles congruent (all 600) – also equilateral

Cuts into 2 equal pieces

Isosceles Triangle

leg leg

base

base angles

•Legs are congruent

•Base angles are congruent

•If we know 1 angle, we can find the rest

Always start byidentifying baseangles and legs!

If one is true, the other must be true!

Answer: A

Multiple-Choice Test Item If and what is the measure of

A. 25 B. 35 C. 50 D. 130

Mark given information first

Identify legs & base angles

Use angle relationships

Equilateral Triangles

•A special type of isosceles triangle

•All sides and angles are congruent

•Also called equiangular

•All angles must be 600

a. Find x.

b.

Answer: 90

Answer: 30

ABC is an equilateral triangle. bisects