f‘(x)=3x^2-2(a+1)x+4a=3[x-(a+1)/3]^2+4a-(a+1)^2 /9
|(a+1)/3|<1, -4
4a-(a+1)^2 /9 = 0
a=17±12√2,
综上,a=17-12 √2, b为任意实数