已知sin(α+π/12)＝1/3,求cos(α+7π/12)的值

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已知sin(α+π/12)＝1/3,求cos(α+7π/12)的值
已知sin(α+π/12)＝1/3,求cos(α+7π/12)的值

已知sin(α+π/12)＝1/3,求cos(α+7π/12)的值
cos(α+7π/12)
=sin(-α+-π/12)
=-sin(α+π/12)
=-1/3

cos(α+7π/12)=cos(α+π/12+π/2)=cos(α+π/12)cos(π/2)-sin(α+π/12)sin(π/2)=-sin(α+π/12)