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Posted by on Friday, December 30, 2011 at 4:52am.

Prove that
sin(n+1)Asin(n+2)A + cos(n+1)Acos(n+2)A = cosA

  • Trigonometry - , Friday, December 30, 2011 at 9:31am

    Use the same method as my other proof, but let
    (n+1)A = x and
    (n+2)A = y
    y-x = A

    sin(n+1)Asin(n+2)A+cos(n+1)Acos(n+2)A
    = sinxsiny + cosxcosy
    = cos(y-x) = cosA

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