(1)令f’(x)=1/x-a=0==>碰卜x=1/a当a>0时,f’’(x)=-1/x^2<0,函数在x=1/a处笑旅穗取极大值;(2)由(1)函数在x=1/镇猜a处取极大值∴f(1/a)=-lna-1 <=-1==>a>=1