sin²20°+sin80°sin40°且sin(a+b)sin(a-b)=sin^2a-sin^2b
来源:学生作业帮 编辑:搜搜做题作业网作业帮 分类:数学作业 时间:2024/07/22 14:16:36
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](/uploads/image/z/17372508-60-8.jpg?t=sin%26%23178%3B20%C2%B0%2Bsin80%C2%B0sin40%C2%B0%E4%B8%94sin%28a%2Bb%29sin%28a-b%29%3Dsin%5E2a-sin%5E2b)
sin²20°+sin80°sin40°
=sin²20°+sin(60°+20°)sin(60°-20°)
=sin²20°+sin²60°-sin²20°
=sin²60°
=3/4
=sin²20°+sin(60°+20°)sin(60°-20°)
=sin²20°+sin²60°-sin²20°
=sin²60°
=3/4
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