## 7.3 Outline

- Terminology
- six parts of a triangle
- classifying triangles
- by sides
- scalene
- isosceles
- equilateral

- by angles
- acute
- right
- obtuse

- by sides
- congruent triangles
- construction
- corresponding parts

- Angles of a triangle
- angles in a triangle
- exterior angle
- exterior angle property

- Isosceles triangle property
- vertex angle
- base angle
- isosceles triangle property
- equiangular implies equilateral

## 7.3 Essential Ideas

Triangles are classified both by sides and by angles:

**By sides**: scalene (no equal sides)

isosceles (two equal sides)

equilateral (three equal sides)

**By angles: **acute (three acute angles)

right (one right angle)

obtuse (one obtuse angle)

**Important Triangle Properties**

The sum of the measures of the angles of a triangle is 180 degrees.

The measure of the exterior angle of a triangle equals the sum of the measures of the two opposite interior angles.

If two sides of a triangle have the same length, then the angles opposite them are equal.