# Circle theorems

Rules about angles inside circles. One example: an angle drawn from the two ends of a diameter to any point on the circle is always 90 degrees.

Maths (school) · Part 05: Angles and area · Card 8 of 8 · Last updated 2026-10-19

## Analogy

Like a rope pulled tight across a circular room: the angle always stays a right angle.

## Example

**Angle in a semicircle**

Triangle on a diameter
One angle is 90
Others: 35 and 55
35 + 55 + 90 = 180

## In practice

Craftspeople use this to find the exact centre of a round object using a set square.

## Practice

### Flashcards

- **What is the angle in a semicircle?** 90 degrees.

## Questions people ask

### What is the angle in a semicircle?

It is always 90 degrees. If you join both ends of a diameter to any other point on the circle, the angle at that point is a right angle. This is one of the most-used circle theorems and it is simple to apply.

### What does the angle at the centre theorem say?

The angle at the centre of a circle is twice the angle at the edge when both are drawn from the same two points. If the angle at the edge is 40 degrees, the angle at the centre is 80 degrees.

## Related concepts

- [Angle rules](https://studyvisually.com/learn/science/maths-school/maths-school-part-5/maths-school-part-5-concept-1)
- [Angles in parallel lines](https://studyvisually.com/learn/science/maths-school/maths-school-part-5/maths-school-part-5-concept-2)
- [Angles in polygons](https://studyvisually.com/learn/science/maths-school/maths-school-part-5/maths-school-part-5-concept-3)
- [Circles](https://studyvisually.com/learn/science/maths-school/maths-school-part-5/maths-school-part-5-concept-5)

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