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prove sin(α+β)sin(α-β)=cos^2β-cos^2α

  • Trig -

    (aina cosb + cosa sinb)(sinacosb - cosa sinb)
    sin^2acos^2b + sinacosasinbcosb - sinacosasinbcosb - cos^2asin^2b)
    (1-cos^2a)cos^2b - (1-cos^2b)cos^2a
    cos^2b - cos^acos^b - cos^2a + cos^2acos^2b
    cos^2b - cos^2a

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