find the coefficent of x^5 in the expansion of
(1-x)^7+x(1-x)^8+x^2(1-x)^9
中四一條amaths.超急
2007-10-15 2:24 am
回答 (2)
2007-10-15 7:15 am
✔ 最佳答案
(1-x)^7+x(1-x)^8+x^2(1-x)^9 =(1-x)^7[(1)+(x)(1-x)+(x^2)(1-x)^2]
=(1-7x+21x^2-35x^3+35x^4-21x^5+7x^6-x^7)[1+x-x^2+(x^2)(1-2x+x^2)]
=(1-7x+21x^2-35x^3+35x^4-21x^5+7x^6-x^7)(1+x-x^2+x^2-2x^3+x^4)
=35x^5+35x^5-42x^5-7x^5-21x^5+...
=0x^5+...
∴Coefficient of x^5 in the expansion is 0﹟
參考: 自己
2007-10-15 2:47 am
哇好難我s.1唔識
sorry
sorry
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