用积化和差做cos10°cos50°cos70°=0.5*{cos(10°+50°)+cos(10°-50°)}*cos70°=0.5(cos60°+cos40°)cos70°=0.5(0.5+cos40°)cos70°=0.25*cos70°+0.5*cos40°cos70°=0.25*cos70°+0.5*0.5{cos(40°+70°)+co...