sin(π/3-α)=sin(π/2-π/6-α)=sin[π/2-(π/6+α)]=cos(π/6+α)=1/4 ①
cos(π/3+2α)=cos2(π/6+α)=2cos²(π/6+α)-1 ②
将①式代入②式得
cos(π/3+2α)=-7/8