Sn=2n/(n+1)
sn-1=2(n-1)/(n-1+1)=2(n-1)/n
Un=Sn-sn-1
=2n/(n+1)-2(n-1)/n
=[2n*n-2(n-1)(n+1)]/n(n+1)
=(2n^2-2n^2+2)/n(n+1)
=2/n(n+1)