Our teacher used the following on a problem solution:(adsbygoogle = window.adsbygoogle || []).push({});

cos(x-a)cos(b-x) = [itex]\frac{1}{2}[/itex][cos(a+b-2x) + cos(a-b)]

Where does this come from? I can't find it in anywhere (except for wolframalpha). Thanks.

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# Where does this trig identiy come from?

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