If x and y are real numbers and SiNx * cosy = 1, then cos (x + y)=
sinx*cosy=1
SiNx = cosy = 1 or SiNx = cosy = - 1
cosx=siny=0
So cos (x + y) = cosxcosy sinxsiny = 0
sinx*cosy=1
SiNx = cosy = 1 or SiNx = cosy = - 1
cosx=siny=0
So cos (x + y) = cosxcosy sinxsiny = 0