Question: Why does nobody talk to circles?

Because there is **no point**!

But within one particular circle theorem, there are **points** in a circle in the shape of a **quadrilateral**.

This circle theorem is known as **angles in a cyclic quadrilateral**.

Let's investigate this theory in action now...

**The sum of the opposite angles in a cyclic quadrilateral add up to 180°. **

So in the diagram above, **a + d = 180°** and **b + c = 180°**.

For example, in the diagram below the **opposite angles **add up to 180°:

76 + 104 = 180°

92 + 88 = 180°

By extension, this means that the **total angles **in a quadrilateral will **add up to 360°**.

But, be careful!

The quadrilateral must fill the circle (i.e. each of its four corners must touch the circumference) or it cannot be classed as a cyclic quadrilateral. |

In this activity, we will find the value of unknown angles and solve problems requiring the application of the theory explained above.

As ever in circle and angle investigations, we will be asked to recall other angle properties too.