"why does the professor derive the equation?" I often hear people of other majors (engineering) ask me why the professor derives the equation to be used in some situation (like a continuous charge distribution electric field from coulomb's law divided by the test charge) instead of just giving them the bottom line formula. This question infuriates me because I don't know how to answer it. I learn how to derive all my equations because I like to. I understand the formula better and can modify it to a situation better because I know how it is formed. I just feel like the question sounds like they don't want to do ANYTHING, I mean anyone can plug numbers into an equation. What's worse is when I help someone solve some intensive problem and end up with some simple expression at the very end like "x = 2q^2" and they are like "cool, thanks" and they write down that last little expression that was derived based on the specific circumstances of the problem. Then they memorize that in hopes that that very same question will be on an exam (with different values.) How do you respond to that question?