(1)f(x)的定义域为 x> 0
f(x)的导数＝ax+1- a - 1/x
ax+1- a - 1/x=0
ax^2+(1- a)x - 1=0
(x- 1)(ax+1)=0
x1=1,x2=- 1/a
- 1/a> 0
所以 a