奇函数
f(0)=0
f（x）=x+m/x²+nx+1
0=m/1
m=0
f（x）=x/x²+nx+1
f(-x)=-x/(x^2-nx+1)=-f(x)=-x/(x^2+nx+1)
x^2-nx+1=x^2+nx+1
2nx=0
n=0