hcrdr06
幼苗
共回答了19个问题采纳率:94.7% 举报
证明:sin(α+ β)sin(α- β)
=[sinαcosβ+cosαsinβ](sinαcosβ-cosαsinβ)
=(sinαcosβ)²-(cosαsinβ)²
=sin²α(1-sin²β)-(1-sin²α)sin²β
=sin²α-sin²β
sin²20°+sin80°sin40°
=sin²20°+sin(60°+20°)sin(60°-20°)
=sin²20°+【sin²60°-sin²20°】
=sin²60°
=3/4
1年前
6