f'(x)=1/x -a
如果a=0,
f'(x)=1/x>0,得到单调递增区间[0,正无穷]
f'(x)=1/x0
f'(x)=1/x-a>0,通分,(ax-1)/x0,通分,(-ax+1)/x>0得到单调递增区间[负无穷,1/a]和[0,正无穷]
f'(x)=1/x-a
f'(x)=1/x -a
如果a=0,
f'(x)=1/x>0,得到单调递增区间[0,正无穷]
f'(x)=1/x0
f'(x)=1/x-a>0,通分,(ax-1)/x0,通分,(-ax+1)/x>0得到单调递增区间[负无穷,1/a]和[0,正无穷]
f'(x)=1/x-a