Hello thereddevilsThere's a theorem - see for instance here - that tells you that in the diagram you have here:(It derives from the fact that the triangles and are similar. Can you see why? Hint: use angles in the same segment.)

So, if we plug in the numbers given:Now (although it doesn't look like it in your diagram) is the mid-point of (and the mid-point of ). So cm. Also is a rectangle. So cm.

cm

radius of circle cm.

Grandad