f.4 a math~急急!

(1+x-ax^2)^6
=[(1+x(1-ax)]^6
=1+6x(1-ax)+15[x(1-ax)]^2+20[x(1-ax)]^3+...
=1+6x(1-ax)+15[x^2(1-ax)^2]+20[x^3]+...
=1+6x(1-ax)+15[x^2(1-2ax)]+20[x^3]+...
=1+6x-6ax^2+15x^2-30ax^3+20x^3+...
=1+6x+(15-6a)x^2+(20-30a)x^3+...
the coefficient of x^2 is 27
15-6a=27
a=2
the coefficient of x^3
=20-30a
=20-30*2
=-40

2006-10-22 19:13:10 補充：
sorrya=-2the coefficient of x^3=20-30a=20-30*(-2)=80