Measurements of Common Angles/Straight Angle

Theorem
The measurement of a straight angle is $180 \degrees$ or $\pi$ radians.

Proof
From $2 \pi$ radians, a full rotation is defined to be $360 \degrees$ or $2 \pi$ radians.

Since lines are straight, it therefore follows that from any point on a line, the angle between one side of the line and the other is one half of a full rotation.

Therefore, the measurement of a straight angle is:
 * $\dfrac {360 \degrees} 2 = 180 \degrees$

or:
 * $\dfrac {2 \pi} 2 = \pi$

Also see

 * Two Angles making Two Right Angles make Straight Line