新坐标应该为（Xcos a-Ysin a ,Xsin a+Ycos a）
证明：
设新坐标为(u,v),从x轴正方向逆时针转向(x,y)的角度为b.
则有x/(x^2+y^2)=cos(b)
y/(x^2+y^2)=sin(b)
u/(x^2+y^2)=cos(a+b)=cos(a)cos(b)-sin(a)sin(b)
=cos(a)*x/(x^2+y^2)-sin(a)*y/(x^2+y^2)
v/(x^2+y^2)=sin(a+b)=sin(a)cos(b)+cos(a)sin(b)
=sin(a)*x/(x^2+y^2)+cos(a)*y/(x^2+y^2)
所以 u=cos(a)*x-sin(a)*y v=sin(a)*x+cos(a)*y
即旋转后的新坐标为(u,v)=( Xcos(a)-Ysin(a) ,Xsin(a)+Ycos(a) )