limit

The sandwich or squeezing principle
Suppose that we have three functions f, g, h, and a real number c that is an accumulation point for the domains of each function. The functions verify the following: f(x) ≤ g(x) ≤ h(x), for every x, except possibly c, in a neighbourhood of c. Suppose furthermore that
圖片參考：http://www.batmath.it/eng/a_limits/img/rules.44.gif
.
But your question is independent of sandwich rule
lim n->infinite summation from n = 1 to infinite n^(1/2) / n^(3/2)
=lim n->infinite summation from n = 1 to infinite n^(-1)
=infinite
[Using the ration test
(n+1)/n=1+1/n which is bigger than 1]