Prove that CosB-cosA/cosA+cosB=tanA+B/2tanA-B/2

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  1. use your sum-to-product formulas:

    cosB-cosA = 2 sin (A+B)/2 sin (A-B)/2
    cosA+cosB = 2 cos (A+B)/2 cos (A-B)/2

    now just divide

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