Supplementary Angles

Two Angles are Supplementary if they add up to 180 degrees.


These two angles (140° and 40°) are Supplementary Angles, because they add up to 180°.

Notice that together they make a straight angle.

   

But the angles don't have to be together.

These two are supplementary because 60° + 120° = 180°


Play With It ...

 

If the two angles add to 180°, we say they "Supplement" each other.
Supplement
comes from Latin supplere, to complete or "supply" what is needed.
   
Spelling: be careful, it is not "Supplimentary Angle" (with an "i")

Complementary vs Supplementary

A related idea is Complementary Angles, they add up to 90°

How can you remember which is which? Easy! Think:

  • "C" of Complementary stands for "Corner" right angle (a Right Angle), and
  • "S" of Supplementary stands for "Straight" (180 degrees is a straight line)

You could also think "Supplement" (like a Vitamin Supplement) is something extra, so it it bigger.