Inspired by this very concise answer, which proves that $$\sin^2(\theta)+\cos^2(\theta) \equiv 1 $$ as follows:
then it's simple to see that $$f'(\theta)=0,$$ then $$f(\theta)=f(0)=1,$$
I'm looking for theorems (and corollaries and lemmata) with one-line proofs.
1) The proof of the theorem has to actually deduce something; it can't be, for example, a definition.
2) No vacuous truths; e.g. "If the sky is green, then the Riemann Hypothesis is true" is not allowed.
3) The words "trivially true" are banned!
4) The proof (when rendered in $\LaTeX$), must be under 100 characters, and must be no longer than one sentence.
If you could state the theorem along with its short proof, that would be great!
Obviously, there are lots of theorems with proofs of varying lengths-- I just care about the short ones!