已知a+b=3,ab=2 ,求21a3b+a2b2+21ab3 的值

21a^3b+a^2b^2+21ab^3
=ab(21a^2+ab+21b^2)
=42(a^2+b^2)+4
=42((a+b)^2-2ab)+4
=42(3^2-4)+4
=42*5+4
=214

21a3b+a2b2+21ab3=21ab(a^2+b^2)+a^2b^2
=21ab(a^2+b^2+2ab-2ab))+a^2b^2
=21ab(a+b)^2-41a^2b^2
=21*2*3^2-41*(2)^2
=378-164
= 214