Proving Product-to-Sum Identities: Need help

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Homework Statement


Prove the following product-to-sum identities:
i) cosucosv = 1/2[cos(u + v)+cos(u - v)]

ii) sinusinv = 1/2[cos(u - v) - cos(u + v)]

Any help/hints would be appreciated. The TAs (teacher assistants) can't help us with our problem set questions, so I'm stuck on this one. I really suck at proving identities. :(
 
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You must have sum and difference formulas for cos(u+v) and cos(u-v), right? Use them.
 
What do you mean by sum and difference formulas?
 
cos(u+v)=cos(u)cos(v)-sin(u)sin(v). What's cos(u-v)?
 

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