Elementary Geometry for College Students (7th Edition)

When $\angle AOB$ is subdivided into a certain number of angles, the sum of these angles must be $180^{\circ}$
Since AB is a straight line, the angle $\angle AOB$ is equal to $180^{\circ}$ When $\angle AOB$ is subdivided into a certain number of angles, the sum of these angles must be $180^{\circ}$