sin20乘sin40乘sin80,等,急用……!
问题描述：
sin20乘sin40乘sin80,等,急用……!
答
sin20°sin40°sin80°=sin40°*[2sin20°sin80°]/2=sin40°*[cos(20°-80°)-cos(20°+80°)]/2
=sin40cos60+sin40°cos80°=(sin40°)/2 +[sin(40°+80°)+sin(40°-80°)]/2
=(sin40°)/2 +(sin120°)/2+-(sin40°)/2=√3/4；
答
可以使用积化和差公式求解.刚才解答有误,抱歉.sin20°sin40°sin80° =-(1/2)[cos60°-cos(-20°)]sin80° =-(1/4)sin80°+(1/2)cos20°*sin80° =-(1/4)sin80°+(1/2)*(1/2)(sin100°+sin60°) =-(1/4)sin80°+(1/4...