Supplementary angle

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See also the animated version.

After doing this Your work should look like this
Start with a angle BAC.
1.  Extend either leg (here AC) backwards, away from the interior of the angle.
Done.  The angle ∠DAB and the given angle ∠BAC are supplementary.
(I.E. they add to 180°)
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Non-Euclidean constructions