3.13 8 sin 120° cos 660°

8 * sin(120°) * cos(660°) = 8 * sin(180°-60°) * cos(660°-360°) = 8 * sin(60°) * cos(300°) = 8 * (√3/2) * cos(360° - 60°) = 8 * (√3/2) * cos(60°) = 8 * (√3/2) * (1/2) = 2√3