函数f(x)=x 的绝对值/(X-1)-kx
x>=0 f(x)=x/(x-1)-kx=[－kx^2+(1＋k)x]/(x-1)
x1=0 x2=(k＋1)/k>0A={k|k>0或k<-1}
x<=0f(x)=-x/(x-1)-kx=[-kx^2+(k-1)x]/(x-1)
x1=0 x2=(k-1)/k<0 B={k|0k=0,1,-1时,有一个零点
0k>0 或k<-1有两个零点