There are different patterns where you can use another sequence Y instead of X.
The best is to post here your exact sequence X, so anybody could give you some tips around it.
Quote (Mastersam93 @ Apr 2 2015 03:09pm)
So, for some sequence X, to prove lim(X) = L I have to prove | Xn - L | < ε for all ε > 0 and n > M
(...)
Notice this exact phrasing for X to tend towards L :
for all ε > 0, a rank M exists such that | Xn - L | < ε for all n > M
It must be clear that M depends on ε.