对f(x)求导
f'(x)=2ax+2+b/x
x=1和x=2取极值,很显然,代入f'(x)=2ax+2+b/x等0即
2a+2+b=0
4a+2+b/2=0
联立,解得,a=-1/3,b=-4/3
2)f(x)=-1/3x^2+2x-4/3*lnx
f(1/2)=11/12+4/3ln2
f(1)=5/3
f(2)=8/3-4/3ln2
所以,最小值为5/3,最达值为11/12+4/3ln2.