a²-6a+9=(a-3)^2≥0
Ib-1I≥0,与a²-6a+9相反数,故
Ib-1I=0,b=1
(a-3)^2=0,a=3
a³b³+2a²b²+ab=ab(a^2b^2+2ab+1)=ab(ab+1)^2=3*a^2=48