f(x)=[(x+1)(x+a)]/x为奇函数
f(-x)=-f(x)
(-x+1)(-x+a)/(-x)=-[(x+1)(x+a)]/x
(1-x)(a-x)/x=[(x+1)(x+a)]/x
(1-x)(a-x)=(x+1)(x+a)
x^2-(a+1)x+a=x^2+(a+1)a+a
所以,a+1=0
a=-1