f(x)=1/2ax2 +lnx
f'(x)=ax +1/x
定义域x＞0
当a＞0时
f'(x)=ax +1/x ＞0
函数（0；+无穷）递增
当a＜0时
f'(x)=ax +1/x =(ax^2+1)/x＞0
ax^2+1＞0
x＜√-a/a
函数（0；√-a/a）递增；函数（√-a/a；+无穷）递减