y=根号下（sin^2(x/2)-2sin(x/2)cos(x/2)+cos^2(x/2)）+根号下（sin^2(x/2)+2sin(x/2)cos(x/2)+cos^2(x/2))=根号下【（sin（x/2）-cos（x/2））^2】+根号下【（sin（x/2）+cos（x/2））^2】=|sin(x/2)-cos(x/2)|+|si...为什么是2/x二倍角公式啊sinx=sin2(x/2)=2sin(x/2)cos(x/2)