I'm having a lot of problems with questions that involve double angles and half angles... Can someone help me understand how to solve two types of problems?

The first one is: ~Use the following information to find sin 2A, cos 2A, and tan 2A. cos A = 5/13, in quadrant 4.~

The second one is: ~Use the following information to find cos A/2, sin A/2, and tan A/2. There is a right triangle and the leg opposite angle A is 8 and the adjacent leg to angle A is 15.~

There are a lot of questions like these two in my homework. Can someone please tell me how to do them? I have all the formulas in my text book, but I still don't understand how to do it.

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