# Complementary Angles

Two angles are Complementary when 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°

## Play With It ...

(Drag the points)

### 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.

When 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

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

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

- Complementary add to 90°
- Supplementary add to 180°

You can also think:

- "
**C**" of**C**omplementary is for "**C**orner" (a Right Angle), and - "
**S**" of**S**upplementary is for "**S**traight" (180° is a straight line)

Or you can think:

- when you are
**right**you get a**compliment**(sounds like compl**e**ment) - "supplement" (like a vitamin supplement) is something extra, so is bigger