# Section 7.3: Triangles

## 7.3 Outline

1. Terminology
1. six parts of a triangle
2. classifying triangles
1. by sides
1. scalene
2. isosceles
3. equilateral
2. by angles
1. acute
2. right
3. obtuse
3. congruent triangles
1. construction
2. corresponding parts
2. Angles of a triangle
1. angles in a triangle
2. exterior angle
3. exterior angle property
3. Isosceles triangle property
1. vertex angle
2. base angle
3. isosceles triangle property
4. 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.