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展開式中第二項、第三項、第四項係數的絕對值分別為n,n(n-1)/2,n(n-1)(n-2)/6,由已知可得n+n(n-1)(n-2)/6=n(n-1),兩端除以n得1+(n-1)(n-2)/6=n-1,化簡得n^2-9n+14=0,解得n=7或n= 2(舍去)展開共8項…
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