Albert Teen YOU ARE LEARNING:  Rules of Angles  # 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

1

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

Here is an example 2

A right angle is exactly $90\degree$

We indicate a right-angle with a little square at the angle. 3

What type of angle is this?  4

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

Both these angles are obtuse 5

A reflex angle is greater than $180\degree$

Both these angles are reflex angles 6

What type of angle is this?  7

This is a reflex angle

We can see this because the angle is greater than $180\degree$ 8

What type of angle is this?  9

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

1

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

No matter what the two angles are, they always add up to $180\degree$ 2

What is the value of angle B? Write the number of degrees.  3

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

No matter how many angles there are, they all add up to $360\degree$ 4

What is the size of Angle A? Write the number of degrees  5

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

Have a look in this triangle to see for yourself 6

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? 