**Angles** have various** properties** or facts which we can use to help us solve related problems.

In this activity, we will be investigating and utilising this fact:

**Angles on a straight line will always add up to 180°.**

It is important to note, this condition about the placement of the angles:

Let's look at this information in practice in some examples now.

**e.g. Find the missing angle in the diagram below.**

We know that the sum of the angles on the line **must **add up to **180°**.

Therefore, we can calculate the missing angle by subtracting the known angle from 180:

180° - 45° = **135° **

That wasn't too tricky, was it?

Let's try another to check you have the rule down and know how to apply it accurately.

**e.g. Find the angle x in the diagram below.**

We might have been given two angles this time, but that is no bother, we just need to follow the same process through **twice**:

180° - 28° - 53° =** ****99°**

Alternatively, we can add the angles given together **first **and then subtract this total from 180°:

28° + 53° = 81°

180° - 81° = **99° **

Right then, let's put what we know into action now.

In this activity, we will use the key fact, that angles on a line always add to 180°, to find the value of unknown angles using numbers and algebra and solve problems involving collections of angles.