已知sin(π/3 +α)=-1/3,则cos(7π/6 -α)=?
问题描述：
已知sin(π/3 +α)=-1/3,则cos(7π/6 -α)=?
答
cos(7π/6 -α)
=cos(π+π/6 -α)
=-cos(π/6 -α)
=-cos(π/2-π/3-α)
=-sin(π/3+α)
=-(-1/3)
=1/3