Complementary Angles

Two Angles are Complementary if they add up to 90 degrees (a Right Angle).

These two angles (40° and 50°) are Complementary Angles, because they add up to 90°.

Notice that together they make a right angle.

But the angles don't have to be together.

These two are complementary because 27° + 63° = 90°

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Right Angled Triangle

In a right angled triangle, the two non-right angles are complementary, because in a triangle the three angles add to 180°, and 90° has already been taken by the right angle.

	If two angles add to 90°, we say they "Complement" each other. Complementary comes from Latin completum meaning "completed" ... because the right angle is thought of as being a complete angle.

	Spelling: be careful, it is not "Complimentary Angle" (with an "i") ... that would be an angle you get for free!

Complementary vs Supplementary

Note: A related idea is Supplementary Angles - those add up to 180°

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

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

Or you can think: when you are right you get a compliment (sounds like complement).

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