当n=km,m为无限大,x有限时,上式可化简为:
y=1-[n(n-1)(n-2)...(n-x)]/[m(m-1)...(m-x)]
=1-[km(km-1)...(km-x)]/[m(m-1)...(m-x)]
=1-k^(x+1)