The following Nrich weekly problem: http://nrich.maths.org/7050 asks the solver to find a trig identity using some circle arithmetic. The solution apparently "says it all", but to my mind, it's not saying it loud enough. I got sin2A = 2sinAcosA from the original diagram (but I had to use the sine rule and pythag to do so); the proposed solution does not mention the identity they have in mind, and also has modified the original geometry a little for some reason. Could any tell me what identity they are aiming for, and what the full derivation is. I'm clearly to dimwitted to see the obvious solution they have in mind.