[aninhumer]

>it is really inefficient to derive things from scratch every time.

Well yeah, it would be really inefficient to derive the formula and then plug numbers into it, but you don't have to do that, you can just teach people to complete the square instead. It's pretty much as fast as using the formula (since it's the same operations), but it builds on your existing equation manipulation skills and so you understand every single step. And if you ever need to actually know the equation for whatever reason, you can derive it easily.