Trig Identity

Here are some things you can try:

It's a good idea to draw out the diagram and include on it the facts that you know (such as the radius and the angle marked $A$).

The question mentions $2A$.  Can you find an angle on your diagram which is equal to $2A$? (Hint - circle theorems may be useful!).

Trigonometry is also mentioned.  Can you find a length equal to $\cos A$ or $\sin A$?  It might be helpful to find some right-angled triangles.

The diagram below has some extra lines drawn on creating some right-angled triangles.