y=(cx+d)/(ax+b)
=(cx+bc/a+d-bc/a)/[a(x+b/a)]
=c/a+(ad-bc/a)/a^2 *1/(x+b/a)
=c/a+k/(x+b/a),这里 k=(ad-bc)/a^2
定义域为x>-b/a,及x0时(即ad-bc>0),y在两个定义域区间x>-b/a,及x