The definition of a convergent squence is:

For every L in Real numbers, and all E>0 such that there exists N in Natural numbers n>/= N implies |xn-L|>E.

Negate this statement then use the negation to prove that the sequence xn=(-1)^n is not convergent.

I got the negation I think, but I'm not sure how to begin with the proof. We went over one example in class and it was for proving something converges. I got the negation as :

For all L in real numbers, there exists E>0 such that there exits N in natural numbers and n>/=N implies |xn-L|<E