Supplementary Angles

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


supplementary angles 40 and 140

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

Notice that together they make a straight angle.


supplementary angles 60 and 120

But the angles don't have to be together.

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

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When the two angles add to 180°, we say they "Supplement" each other.

comes from Latin supplere, to complete or "supply" what is needed

Spelling: be careful, it is not "Supplimentary Angle"

Complementary vs Supplementary

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

How to remember which is which? Well, alphabetically they are:

  • Complementary add to 90°
  • Supplementary add to 180°

You can also think:

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

Or you can think:

  • when you are right you get a compliment (sounds like complement)
  • "supplement" (like a vitamin supplement) is something extra, so is bigger


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