**Question: **What do triangles and angles on a straight line have in common?

**Answer : **Angles in a triangle and angles on a straight line

**always add up to 180°**.

Here is a further fact relating to angles and straight lines:

**Interior angles** are found within parallel lines, on the **same side** of the transversal (or the line drawn through them).

These angles are sometimes called **co-interior** or **supplementary **angles, and we need to be prepared to recognise them by any of these names.

There are very few numbers which have great significance in maths, but **180** is definitely one of them!

Great stuff - three facts for the price of one!

To calculate a **missing interior angle**, we just need to subtract the angle given from 180°.

Let's put this knowledge of supplementary and interior angles into action now.

In this activity, we will learn how to recognise all types of supplementary and interior angles and use the key fact, that interior angles add up to 180°, to find the value of unknown angles and solve related problems.