Manipulate[With[{g=f/.n->m},Text@TraditionalForm@Pane[g==ReleaseHold[g],400]],{{m,25,"n"},1,50,1},{{f,HoldForm[Sum[k^n 3^k,k]],"sum"},{HoldForm[Sum[k^n,k]],HoldForm[Sum[k^n 3^k,k]],HoldForm[Sum[(1/(k(k+n))),k]],HoldForm[Sum[((2^k)/(k(k+n))),k]]}}]