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