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.
This is the same drawing as the last step in the above animation.
|1||m∠DAB + m∠BAC = 180°||A linear pair, so add to 180°|
|2||∠DAB is the supplementary angle to ∠BAC||From (1)|
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.