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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°

Right Angled Triangle

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


  say If the two angles add to 90°, we say they "Complement" each other.

comes from Latin completum meaning "completed" ... because the right angle is thought of as being a complete (full) angle.
  spell 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" right angle(a Right Angle), and
  • "S" of Supplementary stands for "Straight" (180 degrees is a straight line)