A1/A2=1/2A2/A3=3/2....A(n-1)/A(n-2)=(n-2)/(n-1)An/A(n-1)=(n-1)/n叠乘得An=1/n
A(n+1) = n/n+1 * n-1/n * .... 1/2 * 1=1/n+1
A(n)=1/n...