Supplementary Angles

Note the angles AÔB and BÔC in the figure below:

The half straight form a shallow angle. Verify that:

m (AÔB) + m (BÔC) = 180º

In this case we say that the angles AÔB and BÔC are supplementary. Like this:

Two angles are supplementary when the sum of your measurements is 180º.


The angles measuring 82º and 98º are supplementary, since 82º + 98º = 180º.

We say that the angle of 82º is the supplement of the angle of 98º, and vice versa.

To calculate the supplement measure From an angle, we must determine the difference between 180º and the measurement of the given acute angle.

Angle measurement Supplement
x 180º - x


  • How much does an angle supplement of 55º measure?


Supplement Measurement = 180º - Angle Measurement

Supplement Measure = 180º - 55º

Supplement Measurement = 125º

Therefore, the supplemental angle measurement of 55º is 125º.


The angles XÔY and YÔZ of the figure below, besides being supplementary, are also adjacent. We say these angles are additional adjoining.

Next: Vertex Opposite Angles