(1) 将x1=3,x2=4分别代入方程f(x)-x+12=0
9/(3a+b)-3+12=0,3a+b=-1
16/(4a+b)-4+12=0,4a+b=-2
解方程组得：a=-1,b=2
f(x)=x^2/(ax+b)=x^2/(2-x)
(2).f(x)(k+1)x-k
x^2-(k+1)x+k>0
(x-k)(x-1)>0
k=1时,解为x不等于1
k2
2>=k>1时,解为x>2
k>2时,解为x>k
x