помогите! ! 1. cos 7 пи / 4. 2. как упростить sina*cos*a*ctga-1

1)
cos(7pi/4) = cos(2pi - pi/4) = cos(pi/4) = V2/2
2)
... = sin(a)*cos(a)*cos(a)/sin(a) - 1 = cos^2(a) - 1 = -(1 - cos^2(a)) = -sin^2(a)