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Trig Identity
Age 16 to 18
Challenge Level 
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.
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A Close Match
Can you massage the parameters of these curves to make them match as closely as possible?
