sin²20°+sin80°sin40°且sin(a+b)sin(a-b)=sin^2a-sin^2b

来源：学生作业学帮网编辑：学帮网时间：2024/05/27 02:40:22

sin²20°+sin80°sin40°
=sin²20°+sin（60°+20°）sin（60°-20°）
=sin²20°+sin²60°-sin²20°
=sin²60°
=3/4