Hi again. Yes, I do fully understand why the output was -1 in my case. Clearly, that’s coming from the assumption that the variable b is an independent variable. But is it? Clearly, it’s not and that’s my principal concern. Isn’t it wrong to assume that by simply replacing the product xy with b in the equation f = x + y + xy, b becomes independent of x and y?
One can be more careful, and say that f = b/y + b/x + b, so df/db = 1/y + 1/x + 1. Still, by implying that d(b/y)/db = 1/y, we would have assumed that y is a constant – why should that be true?