如图所示
sin(π/6 - α)=sin(π/2 - π/3 - α)=sin[π/2 - (π/3 + α)]=cos(π/3 + α)=1/3则cos(2π/3 + 2α)=cos[2(π/3 + α)]=2cos²(π/3 + α) - 1=2•(1/3)² - 1=2/9 - 1=-7/9
这样的看看行不行
