C(n-1,2n)=[2n×(2n-1)×(2n-2)×(2n-3)×.×(n+3)×(n+2)]/(n-1)!
C(n,2(n-1) )=[(2n-2)×(2n-3)×.×(n+1)×n×(n-1)]/n!
C(n-1,2n)/C(n,2(n-1) )=2n×（2n-1）×n/(n+1)×n×(n-1)=(4n^2-2n)/(n^2-1)
4n^2-2n=56
n^2-1=15
n=4