User:Leigh.Samphier/Sandbox/Matroid Satisfies Base Axiom/Sufficient Condition/Lemma/Lemma 1

Theoren
Let $U, V$ be finite sets.

Let $\card V < \card U$.

Then:
 * $\card{V \setminus U} < \card{U \setminus V}$

Proof
We have: