Question

sin(n+1)x sin(n+2)x+cos(n+1)x cos(n+2)x

Answer 1

sin(n+1)xsin(n+2)x+cos(n+1)xcos(n+2)x
[cos(a-b)=cosacosb+sinasinb]
cos[(n+1)x-(n+2)x]
cos[nx+x-nx-2x]
cos(-x)
[cos(-x)=cosx]
cosx