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Angles On One Side of A Straight Line

Angles on one side of a straight line always add to 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 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° 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°