Hi kozy,

For your case, since you're only changing the variable D, it is a 1 variate equation. A B and C can be treated as a constant.

If differentiating is an issue, consider using a "proxy" for differentiation as the change in the function for a small change in D.

I.e. f'(D) = f(D) - f(D + delta) where delta is a very small number (the smaller the more accurate it is)