设cosα=√3/3,0°<α<90°,cos(A-30°)=cosα,A-30°=α,A=30°+α,2A+30°=60°+2α+30°=90°+2α,sin(2A+30°)=sin(90°+2α)=sin(90°-2α)=cos(2α)=2(cosα)^2-1=2*(√3/3)^2-1=2/3-1=-1/3.
