sin70°cos20°-sin10°sin50°的值是多少?,麻烦给出过程
来源:学生作业帮 编辑:作业帮 分类:数学作业 时间:2024/04/27 23:34:49
sin70°cos20°-sin10°sin50°的值是多少?,麻烦给出过程
∵积化和差公式 sinαcosβ=[sin(α+β)+sin(α-β)]/2
sinαsinβ=-[cos(α+β)-cos(α-β)]/2
∴sin70°cos20°-sin10°sin50°
=[sin(70°+20°)+sin(70°-20°)]/2-{-[cos(10°+50°)-cos(10°-50°)]}/2
=[sin90°+sin50°+cos60°-cos(-40°)]/2
=[1+sin50°+1/2-cos40°]/2
=[1+cos40°+1/2-cos40°]/2
=(1/2)(3/2)
=3/4
sinαsinβ=-[cos(α+β)-cos(α-β)]/2
∴sin70°cos20°-sin10°sin50°
=[sin(70°+20°)+sin(70°-20°)]/2-{-[cos(10°+50°)-cos(10°-50°)]}/2
=[sin90°+sin50°+cos60°-cos(-40°)]/2
=[1+sin50°+1/2-cos40°]/2
=[1+cos40°+1/2-cos40°]/2
=(1/2)(3/2)
=3/4
sin70°cos20°-sin10°sin50°的值是多少?,麻烦给出过程
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