Circle theorems · Year 10
In Year 10 you learn: angles in circles.
Circle theorems are facts about the angles formed by chords, radii and tangents of a circle. They come from isosceles triangles (two radii are always equal) and are used, with reasons, to find missing angles.
Key facts
- The angle at the centre is twice the angle at the circumference standing on the same arc.
- The angle in a semicircle is 90°.
- Opposite angles of a cyclic quadrilateral add up to 180°.