Trigonometry

Prove that
sin(n+1)Asin(n-1)A + cos(n+1)Acos(n-1)A = cos2A

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asked by Kewal
  1. Let (n+1)A = x
    Let (n-1)A = y
    sinx*siny + cosx*cosy = cos (x - y)
    (This is an identity you should know)
    Now note that x -y = 2A , so

    sinx*siny + cosx*cosy = cos (x - y)
    = cos(2A)

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    posted by drwls

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