Hello,
Well, do you know identities for cos(a+b) and cos(a-b) ?
cos(a+b)=cos(a)cos(b)-sin(a)sin(b)
So what will cos(a-b) be ?
For the second one, use twice the formula for cos(a+b) :
cos(3x)=cos(2x+x)=...
Add the equations together.
If A = B:
Convert into terms of only cosA and you should get your answer
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Use your result from the first question to split your products in to sums. It'll be easier to integrate.
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Make the subs:
etc.
You'll need your half-angle identity from your first question to do it after you simplify.