cos(2π/3+2α)=-cos（π-(2π/3+2α)）

　　=-cos(π/3-2α)

　　=-cos(2α-π/3)(公式cos2α=1-2(sinα)^2）

　　=-(1-2(sin(α-π/6))^2)

　　因为sin(π/6-α)=1/3所以（sin(α-π/6))^2=1/9

　　故cos(2π/3+2α)=-(1-2*1/9)

　　=-7/9

　　)=-cos（π-(2π/3+2α)）=-cos(π/3-2α)这一步是如何变化过来的=-cos(2α-π/3)这一步是如何变化过来的

　　cos(2π/3+2α)=-cos（π-(2π/3+2α)）//注：直接将括号内的展开就可以得到下面的=-cos(π/3-2α)//注：根据公式cos(-β)=cosβ,这里有cos(π/3-2α)=cos(-(2α-π/3))=cos(2α-π/3）=-cos(2α-π/3)(公式cos2α=1-2(sinα)^2）=-(1-2(sin(α-π/6))^2)因为sin(π/6-α)=1/3所以（sin(α-π/6))^2=1/9故cos(2π/3+2α)=-(1-2*1/9)=-7/9