I've just been asked for a good example of a situation in maths where using infinity can greatly shorten an argument. The person who wants the example wants it as part of a presentation to the general public. The best I could think of was Goodstein sequences: if you take a particular instance of Goodstein's theorem, then the shortest proof in Peano arithmetic will be absurdly long unless the instance is very very small, but using ordinals one has a lovely short proof.

My question is this: does anyone have a more down-to-earth example? It doesn't have to be one where you can rigorously prove that using infinity hugely shortens the shortest proof. Just something where using infinity is very convenient even though the problem itself is finite. (This is related to the question asked earlier about whether finite mathematics needs the axiom of infinity, but it is not quite the same.)

~~A quick meta-question to add: when I finally got round to registering for this site, I lost the hard-earned reputation I had gained as a non-registered user. I am now disgraced, so to speak. Is that just my tough luck?~~