f(x)=alnx+(ax^2)/2-2x
当a=1时,
f(x)=lnx+x^2/2-2x
f'(x)=1/x+x-1
f''(x)=1-1/x^2
即1-1/x^2即x=1或x=-1时,f(x)存在拐点,即有极值
1-1/x^2>0即x>1或x