A1=1=2/2
An+1/An=n/n+1
An+1=n/(n+1)*An
A2=2/(2+1)*A1
=2/(2+1)*1
=2/3
A3=3/(3+1)*(2/3)
=1/2=4/2
A4=4/(4+1)*(1/2)
=2/5
所以,An=2/(n+1)