Problem on Power of signal 1.1

Ask

Power in the signal s(t) = 8 cos(20*pi-pi/2) + 4 sin(15*pi*t) is

(a) 40                 (b) 41

(c) 42                 (d) 82

Answer is ( a )

Concept

Power in the signal = [(Ac1)^2+(Ac2)^2]/2

                                = (8^2+4^2)/2 = (64+16)/2

                                = 80/2 = 40