# Circle theorems

Source: https://projectgcse.co.uk/maths/shape_space/circle_theorems

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![](https://content.projectgcse.co.uk/img/maths/ct1.gif)

The angle at the centre is twice the angle at the circumference for angles which stand on the same arc.

![](https://content.projectgcse.co.uk/img/maths/ct2.gif)

Angles at the circumference which are in the same segment are all equal.

![](https://content.projectgcse.co.uk/img/maths/ct3.gif)

An angle at the circumference which stands on the diameter of the circle is always a right-angle.

![](https://content.projectgcse.co.uk/img/maths/ct4.gif)

Opposite angles of a cyclic quadrilateral (all vertices on the circumference) add up to 180° (d+f = e+g = 180).

![](https://content.projectgcse.co.uk/img/maths/ct5.gif)

A tangent meets the radius of a circle at 90°.

**There is one more circle theorem, what is it?**