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

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

- Trigonometry -
**drwls**, Friday, December 30, 2011 at 6:08amLet (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)