Complementary angle

This is the step-by-step, printable version. If you PRINT this page, any ads will not be printed.

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.
2.  With the compass at any convenient width, make an arc each side of A, creating points P and Q.
3.  Make the compass width wider, and from P make an arc above A
4.  Repeat from Q, creating the point F above A.
5.  Draw a line from A, up through F
Done.  The angle ∠FAB and the given angle ∠BAC are complementary.
(I.E. they add to 90°)
While you are here..

... I have a small favor to ask. Over the years we have used advertising to support the site so it can remain free for everyone. However, advertising revenue is falling and I have always hated the ads. So, would you go to Patreon and become a patron of the site? When we reach the goal I will remove all advertising from the site.

It only takes a minute and any amount would be greatly appreciated. Thank you for considering it!   – John Page

Become a patron of the site at

Other constructions pages on this site




Right triangles

Triangle Centers

Circles, Arcs and Ellipses


Non-Euclidean constructions