Hello solarscottIn addition to well-known things about isoceles triangles and parallel lines, there are two key facts you need to know:

- Two equal arcs will subtend equal angles at the centre of the circle. And so will two chords of equal length. So if we can prove the chords and are equal in length we're there.

- The exterior angle of a cyclic quadrilateral is equal to the interior opposite angle. So in this case .

OK, so now all you need to prove is that:

- is isosceles

- is isosceles

which you can do by finding some more angles equal to . Then look at the equal length sides of these two triangles, and you should see why .

Can you put all this together now?

Grandad