(20分)Help~中六數統binomial一問。

我本身冇讀開A maths,依家讀數統~但書上有一條例題,我睇極都唔明,所以想大家幫我解一解~

Q: Expand(1+kx- 2x^2)^6 in ascending powers of x as far as the term
in x^2


A: (1+kx- 2x^2)^6
=[1+x(k-2x)]^6
=1+(6C1)[x(k-2x)]+(6C2)[x(k-2x)]^2+...
=1+6kx-12x^2+15[x^2(k^2-4kx+4x^2)]+...
=1+6kx+(15k^2-12)x^2+...


我完全睇唔明anser最尾果兩行,點解最尾果行會簡化成：
=1+6kx+(15k^2-12)x^2+... ??


希望大家可再列詳細d既步驟比我,等我可以知道最尾果行係點得黎~