sin²20°+sin80°sin40°且sin(a+b)sin(a-b)=sin^2a-sin^2b
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sin²20°+sin80°sin40°且sin(a+b)sin(a-b)=sin^2a-sin^2bsin²20°+sin80°sin40°且sin(a+b)sin(a-b)=s
sin²20°+sin80°sin40°且sin(a+b)sin(a-b)=sin^2a-sin^2b
sin²20°+sin80°sin40°且sin(a+b)sin(a-b)=sin^2a-sin^2b
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