I am supposed to calculate a limit, but I do not know how to write it:
This question reads like homework, which we try to keep off the forum.
If you have questions on writing equations in Wolfram Language in the future, at least look up the needed functions in the documentation before asking. Both the in-product (Help menu - Wolfram Documentation) and web site (http://reference.wolfram.com/) have search fields into which you can enter the name of the operation you want to perform (for example, "limit" or "sum"). The reference pages on Wolfram Language functions include many examples that should get you started.
Zhan: No failure. You had the right answer. It just looked odd (well, not odd but rather imaginary). You were almost there.
Oh no, I thought I could help you, but I failed. sorry.