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Where does this trig identiy come from?

  1. Apr 12, 2012 #1
    Our teacher used the following on a problem solution:

    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.
    Last edited: Apr 12, 2012
  2. jcsd
  3. Apr 12, 2012 #2


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    Staff: Mentor

  4. Apr 12, 2012 #3
    thanks for the heads up
  5. Apr 12, 2012 #4

    It comes from the known identity for sum of cosines: [itex]\cos \alpha+\cos\beta=2\cos\frac{\alpha+\beta}{2} \cos \frac{\alpha-\beta}{2}[/itex]

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