(sin20°+2sin40)/sin70=?
问题描述：
(sin20°+2sin40)/sin70=?
答
(sin20°+2sin40)/sin70
=[sin(30-10)+2sin(30+10)]/sin70
=[sin30cos10-cos30sin10+2sin30cos10+2cos30sin10]/sin70
=[3sin30cos10+cos30sin10]/sin70
=(3/2*cos10+√3/2*sin10)/sin70
=√3*(√3/2cos10+1/2*sin10)/sin70
=√3*(sin60cos10+cos60sin10)/sin70
=√3sin(60+10)/sin70
=√3.