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