Here is an easy proof.
If it were true that then .
From that we get .
What is wrong to that?
My teacher really takes account about the rigor of a proof. The following is the proposition and a proof given by myself. Can someone please check it whether it is rigorous enough?
Proposition: Let be a sequence that converges to . Show that if for all , then .
Proof: Let , then there exists such that implies . Now take and implies
Since , we must have
Since the difference can be taken arbitrarily small, or even negative. This proves that .
I really doubt the rigor of this proof. And after I type in the proof, I doubt the validity.