an=[(n+1)-n]/n(n+1)
=(n+1)/n(n+1)-n/n(n+1)
=1/n-1/(n+1)
所以S5=1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6
=1-1/6
=5/6