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Find $\cos(x+y)$ if $\sin(x)+\sin(y)= a$ and $\cos(x)+\cos(y)= b$.

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    Jay, you should know that on this site it is considered impolite to issue commands to other users. It would help if you asked kindly, and also explained what you have tried so far, or what your thoughts are about the problem.2011-07-12
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    I've also edited your question so that the equations are in $\LaTeX$.2011-07-12
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    These questions sometimes have many different solutions. Sometimes you just have to play around with things. With sines and cosines, squaring things might help. Then, maybe take advantage of the fact that $\sin^2 u + \cos^2 u = 1$.2011-07-12
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    What have you tried? Have you expanded the left hand side to see if that gives any ideas?2011-07-12
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    It could be a typo, the obvious thing to ask about is $\cos(x-y)$.2011-07-12

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