若sin（6分之派-α）=3分之1,则cos（3分之二派+2α）等于?
问题描述：
若sin（6分之派-α）=3分之1,则cos（3分之二派+2α）等于?
答
cos(2π/3+2α)
=-cos[π-(2π/3+2α)]
=-cos(π/3-2α)
=-cos[2(π/6-α)]
=-{1-2[sin(π/6-α)]^2}
=-(1-2/9)
=-7/9