Angles On One Side of A Straight Line

Angles on one side of a straight 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.


30° + 150° = 180°

 

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

Angle a is 180° − 45° = 135°

 

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

Example: What is angle "b" ?

Angle b is 180° less the sum of the other angles.

Sum of known angles = 45° + 39° + 24°
Sum of known angles = 108°

Angle b = 180° − 108°
Angle b = 72°