Chuckk Hubbard wrote: > i*sin(a) and i*sin(b) multiply to -sin(a)sin(b)? let's forget the sin*sin and look at the i*i part only: since i = sqrt(-1) by definition, i*i is -1 (slowly: i*i= sqrt(-1)*sqrt(-1) = sqrt(-1 * -1) = sqrt((-1)^2) = -1) Thus, i*sin(a) * i*sin(b) = -1 * sin(a)*sin(b) br, piotr