Angles On One Side of A Straight Line

Angles on one side of a straight line always add to 180 degrees

 

angle 180 degrees
30° + 150° = 180°

When a line is split into 2 and we know one angle we can always find the other one.

Example: We know one angle is 45°, what is the other angle "a" ?

angle 45 degrees 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

Angle b is 180° minus 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°