✔ 最佳答案
(1+x-2x^2)^9(1+x)^4
=(1+x(1-2x))^9(1+x)^4
=[1+9x(1-2x)+36x^2(1-2x)^2](1+4x+6x^2+4x^3+x^4)
=1+4x+6x^2+4x^3+9x(1-2x)+9x(1-2x)(4x)+9x(1-2x)(6x^2)+36x^2(1-2x)^2+36x^2(1-2x)^2(4x)+....
=1+4x+6x^2+4x^3+9x-18x^2+36x^2-72x^3+54x^3+36x^2(1-4x+4x^2)+36x^2(1-4x+4x^2)(4x)+....
=1+4x+6x^2+4x^3+9x-18x^2+36x^2-72x^3+54x^3+36x^2-144x^3+144x^3+...
=1+13x+60x^2-14x^3+...
展開式中x^3項的係數=-14
2006-12-04 23:40:35 補充:
sorry﹐做錯漏了9C3x(1-2x)^3(1 x)^4中的x^3項係數展開式中x^3項的係數=-14 9C3=-14 84=70