Hello
Problem:
For which positive integer k is the following series convergent?
The Ratio Test seems a good choice.
After applying the Ratio Test, I will face the following limit:
Then ?
(kn+k)! = (kn+k)...................(kn+1)(kn)!
you have (n+1)^2/[(kn+k)...................(kn+1)
for k = 2
lim(n+1)^2/ [2(n+1)(2n+1)] = 1/4 and we have convergence
For k > 2 limit is 0 as denominator is of order 3 or higher
for k= 1 lim(n+1)^2/(n+1) = infinity