用判别式法：y=(2ax+b)/(x^2+1)yx^2-2ax+y-b=0Δ=4a^2-4y(y-b)≥0y^2-by-a^2≤0故：-1,4是方程y^2-by-a^2=0的根b=3, a^2=4, a=2或-2
