Supplementary angle
Geometry construction using a compass and straightedge

This construction takes a given angle and constructs its supplementary angle. Recall that the supplementary angle is one that makes the given angle become 180°. So an angle of 45° has a supplementary angle of 180° - 45° = 135°.

In this construction you can extend either leg back. It will produce the same result.

## Proof

This is the same drawing as the last step in the above animation.

Argument Reason
1 m∠DAB + m∠BAC = 180° A linear pair, so add to 180°
2 ∠DAB is the supplementary angle to ∠BAC From (1)

- Q.E.D

## Printable step-by-step instructions

The above animation is available as a printable step-by-step instruction sheet, which can be used for making handouts or when a computer is not available.

## Try it yourself

Click here for a printable worksheet containing two supplementary angle angle problems. When you get to the page, use the browser print command to print as many as you wish. The printed output is not copyright.
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