Angles On One Side of A Straight Line

Angles on one side of a staight line will always add to 180 degrees. If a line is split into 2 and you know one angle you can always find the other one.

Example: If we know one angle is 45° what is angle "a" ?

Angle a will be 180° − 45° = 135°

This method can be used for several angles on one side of a straight line.

Example: What is angle "b" ?

In this diagram angle b is simply 180° take away the sum of the other angles. Sum of known angles = 45° + 39° + 24° = 108° Angle b = 180° − 108° Angle b = 72°