As for your code, urgh. A mathematician would carefully define what the function and operands are and the domain of each. More, and I'm vasty simplifying here as I don't have LaTeX available. I don't know what your code does so I've invented some words.
Let f(x,y) represent the rate of change of doodads where x is the number of doodads and y is the amount of doodads per gronk.
f(x,y) = ( x^2 + y^2 ) ^ (1/2)
That's pretty tight and clearer than the code is.