Angle measure is the amount of rotation that it takes to move from one ray of an angle to a second ray. The second ray has a common endpoint with the first ray. This common endpoint is called the vertex of the angle.

A triangle has three interior angles. In the figure below, ∠ABC, shown in blue, ∠BAC, shown in green, and ∠ACB, shown in purple, are the three interior angles of triangle ABC.

In this lesson, you will investigate the relationship between the measures of the three interior angles of any triangle. Then, you will use that relationship to write and solve equations to determine missing angle measures within a triangle.