Derivative of f = (f(x + dx)-f(x))/dx as dx -> 0 Which is to say: 1) create the function f(x + dx) 2) ignore (subtract away) the terms not proportional to dx

Derivative of f = (f(x + dx)-f(x))/dx as dx -> 0 Which is to say: 1) create the function f(x + dx) 2) ignore (subtract away) the terms not proportional to dx 3) of the terms proportional, only care about terms minimally proportional (i.e. divide by dx, keep terms no longer proportional to dx, eliminate any still proportional to dx.) The last step corresponds to eliminating terms proportional to dx^2 or higher