求多项式a^3-5a^2b+3ab^2-3b^3-3ab^2+5ab^2-2a^3的值

将式a^3-5a^2b+3ab^2-3b^3-3ab^2+5ab^2-2a^3
化为
a^3-5a^2b+5ab^2-3b^3-2a^3
=(a^3-3a^2b+3ab^2-b^3)-2a^3+2a^2b+2ab^2-2b^3-4a^2b
=(a-b)^3-2(a-b)a^2+2(a-b)b^2-4a^2b
=(a-b)^3-2(a+b)(a^2-b^2)-4a^2b
=(a-b)^3-2(a+b)(a-b)^2-4a^2b
将a=1,b=2代入上式
得
(1-2)^3-2(1+2)(1-2)^2-4×1^2×2
=-1-6-8
=-15

整理:-a^3+5ab^2+5a^2b-3b^3=-1+20-10-18=-9

=1-10+12-24-12+20-2=-15