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2,3,4,項係數分別為:C(n,1),C(n,2),C(n,3)故有:C(n,1)+C(n,3)=2C(n,2)即n+n(n-1)(n-2)/6=2n(n-1)/2即1+(n-1)(n-2)/6=n-1n^2-3n+2=6n-12n^2-9n+14=0(n-2)(n-7)=0因為n>2,否則沒有第4項所以n=7…
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