若sin(α+π/12)=1/3,则cos(α+7π/12)的值为
问题描述：
若sin(α+π/12)=1/3,则cos(α+7π/12)的值为
答
cos(α+7π/12)
=cos(-α-7π/12)
=sin[π/2-(-α-7π/12)]
=sin(α+13π/12)
=sin[π+(α+π/12)]
=-sin(α+π/12)
=-1/3