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

问题描述:

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

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