y1-y2=1+2m/x1-1-2m/x2=2m(1/x1-1/x2)=2m*(x2-x1)/(x1x2) 因为当X1<0Y2,所以2m*(x2-x1)/(x1x2)>0, 因为x2-x1>0, x1x2<0, 所以m<0
