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)
