Posted by Kewal 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  drwls, 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
yx = A
sin(n+1)Asin(n+2)A+cos(n+1)Acos(n+2)A
= sinxsiny + cosxcosy
= cos(yx) = cosA