a+b=5ab=3a^3.b+2a^2.b^2+ab^3=ab(a^2+2ab+b^2)=ab(a+b)^2=3(25)=75
a^3 b+2a^2b^2+ab^3=ab(a^2+b^2+2ab)=ab(a+b)^2将a+b=5，ab=2代入上式可得=2*（5^2）=50
a^3b+2a^2b^2+ab^3=ab*(a^2+2ab+b^2)=ab*(a+b)^2=3*5^2
