Why do triangle angles always sum to 180°?

In this lesson you will learn about the interior angles of triangles by looking at their

properties.

Let’s Review

The three angles of a triangle always sum to 180°.

98 + 42 + 40 = 18084 + 69 + 27 = 180111 + 46+ 23 = 180

90 + 68 + 22 = 180

Let’s Review

A line that passes through parallel lines is called a transversal.

Let’s Review

Transversals create special angle properties.

Alternate Exterior AnglesCorresponding AnglesAlternate Interior Angles

Core Lesson

Triangles share many of these angle relationships.

1

1 2

3

3

Core Lesson

Find the missing angle measurement.

x

89 + 68 = 157

180-157 = 23

x = 23°

In this lesson you have learned about the interior angles of triangles by looking at their

properties.