Not commenting on the validity of the post, but what is the area of a circle doing there? The only explanation I could see for calculus is if their profits were reported to them in an unclear manner (probably not...?) and then doing an optimization problem, but what kind of information must they be giving you to need calculus rather than using like dimensional analysis or something (or maybe it's a tiered system like tax brackets)

I always just used my left hand with the thumb and pinky as the y and x axes respectively, then the in-between fingers just help me visualize which "point" is higher (we only have two options to choose from if they're not equal anyways)

Absolutely agree, lowk a programmer must've done it else I don't think I've met a mathmematician (or maybe, not crazy enough ones) that ever preferred redefining variables (esp in pedagogical material!!!!)

What do you want to learn PDEs for? It's not my strong point, but I've heard high praise for Partial Differential Equations by Stanley J. Farlow. I found it useful in my undergrad, else I think I could be died in analytical mechanics

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