已知cos(a-π/6)+sina=(4*根号3)/5,则sin(a+7π/6)的值是( )
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已知cos(a-π/6)+sina=(4*根号3)/5,则sin(a+7π/6)的值是( )
A -(2根号3)/5
B (2根号3)/5
C -4/5
D 4/5
A -(2根号3)/5
B (2根号3)/5
C -4/5
D 4/5
![已知cos(a-π/6)+sina=(4*根号3)/5,则sin(a+7π/6)的值是( )](/uploads/image/z/265272-24-2.jpg?t=%E5%B7%B2%E7%9F%A5cos%EF%BC%88a-%CF%80%2F6%EF%BC%89%2Bsina%3D%EF%BC%884%2A%E6%A0%B9%E5%8F%B73%EF%BC%89%2F5%2C%E5%88%99sin%EF%BC%88a%2B7%CF%80%2F6%EF%BC%89%E7%9A%84%E5%80%BC%E6%98%AF%28+%29)
根据:cos(a-π/6)+sina=(4*根号3)/5
有:cosacosπ/6+sinasinπ/6+sina=(4*根号3)/5
(根号3)/2cosa+3/2sina=(4*根号3)/5
得到:cosa+(根号3)sina=8/5
所以:
sin(a+7π/6)=sinacos7π/6+cosasin7π/6=(-根号3)/2sina-1/2cosa
=(-1/2)(cosa+根号3sina)
=(-1/2)*8/5
=-4/5
有:cosacosπ/6+sinasinπ/6+sina=(4*根号3)/5
(根号3)/2cosa+3/2sina=(4*根号3)/5
得到:cosa+(根号3)sina=8/5
所以:
sin(a+7π/6)=sinacos7π/6+cosasin7π/6=(-根号3)/2sina-1/2cosa
=(-1/2)(cosa+根号3sina)
=(-1/2)*8/5
=-4/5
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