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# Rules of Angles

### Rules of Angles

Acute, obtuse, reflex and right-angles include some important rules relative to each other, which we can use to find unknown angles.

Angles can be either **acute**, **right-angled**, **obtuse** or **reflex**. This depends entirely on the **size** of the angle.

Rules of angles

An acute angle is any angle that is less than $90\degree$

Here is an example

A right angle is exactly $90\degree$

We indicate a right-angle with a little square at the angle.

What type of angle is this?

An obtuse angle is between $90\degree$ and $180\degree$

Both these angles are obtuse

A reflex angle is greater than $180\degree$

Both these angles are reflex angles

What type of angle is this?

This is a reflex angle

We can see this because the angle is greater than $180\degree$

What type of angle is this?

This is an obtuse angle

This is because the angle is between $90\degree$ and $180\degree$

There are 3 really important rules to remember about angles.

Rules of angles: lines, circles and triangles

The angles on a straight line add up to $180\degree$

No matter what the two angles are, they always add up to $180\degree$

What is the value of angle B? Write the number of degrees.

Angles around a point add up to $360\degree$

No matter how many angles there are, they all add up to $360\degree$

What is the size of Angle A? Write the number of degrees

The interior angles of a triangle add up to make $180\degree$

Have a look in this triangle to see for yourself

What is the value of angle A? Write the number of degrees.

The angles around a point add up to ____

Final question! Which of these definitions describes the sum of the angles in a triangle?