cosx=cos(x/2 +x/2)=cosx/2 cosx/2 -sinx/2sinx/2=(cosx/2)^2 - (sinx/2)^2=1-(sinx/2)^2 - (sinx/2)^2=1-2(sinx/2)^22(sinx/2)^2=1-cosx(sinx/2)^2=1/2(1-cosx)
COS2X=2COS^2—1=1—2Sin^2用这个公式变来的 要记得哦
