函数f(x)=1/2ax^2-(a+1)x+lnx
故f‘(x)=ax-(a+1)+1/x=(x-1)(ax-1)/x
a∈[1/2,1] 所以1/a∈[1,2]
令f‘(x)=ax-(a+1)+1/x=(x-1)(ax-1)/x>0 得到0