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Cn1+Cn3=2Cn2 n+n(n-1)(n-2)/6=n(n-1) 6+(n-1)(n-2)=6(n-1) n^2-3n+8=6n-6 n^2-9n+14=0 n=2(舍)或n=7 只能先解到這兒,因為沒有二項式.
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