[中學] XY的係數
(X+ 1/X + Y +1/Y)^10 問XY項的係數是多少
小弟的做法是
把XY的能湊成的組合分類 用組合數去求
一、5個X 4個1/X 1個Y C105*C54*C11 =1260
二、4個X 3個1/X 2個Y 1個1/Y C104*C63*C32*C11 =12600
三、3個X 2個1/X 3個Y 2個1/Y C103*C72*C53*C22 =25200
四跟五 就是跟一二X Y相反 所以 直接乘2
我做出來的答案跟解答 的第一+五項 一樣是2520
可是 二三四項卻不對
解答寫的漏漏等
不知道是我錯還是他錯 懇請版上高手解答
解答是1260*2 +8640*2 +21168
如果可以點出我的觀念哪邊有問題更感謝~~~
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