✔ 最佳答案
Sol
n>=7
a(5)=c(n,4)=n!/[(n-4)!4!]=n(n-1)(n-2)(n-3)/24
a(6)=c(n,5)=n!/[(n-5)!5!]=n(n-1)(n-2)(n-3)(n-4)/120
a(7)=c(n,6)=n!/[(n-6)!6!]=n(n-1)(n-2)(n-3)(n-4)(n-5)/720
a(5)+a(7)=2a(6)
1/24+(n-4)(n-5)/720=2(n-4)/120
30+(n^2-9n+20)=12(n-4)
n^2-21n+98=0
(n-7)(n-14)=0
n=7 or n=14