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Math Help - Sum and difference formula

  1. #1
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    Sum and difference formula

    Stuck on this question:

    By using the sum and different formulas for cos(phi+theta) and cos(phi-theta) with phi=1/2(A+B) and theta=1/2(A-B),

    (i) derive the formula
    cos A+cos B=2 cos(1/2(A+B))cos(1/2(A-B))

    (ii) and then must I derive the formula
    cos A-cos B= -2 sin(1/2(A+B))sin(1/2(A-B)) from part (i) above.

    Thanks!
    Brigitte
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  2. #2
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    It's ok. I have managed to solve this now. It was much easier than I had been thinking!
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  3. #3
    MHF Contributor

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    Excellent!
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