(1)先求导f'(x)=1/x+2x+a ,将x=1/2代进去,令f'(x)=0,求得a=-3
(2)先求导f'(x)=1/x+2x+a,其定义域为x大于0,再令f'(x)大于0,得a大于-(1+2x²)/x,设h(x)=-(1+2x²)/x,求h(x)的最大值为x=根号2/2时取得2根号2,所以,a的取值为a大于2根号2