If A = x y (if the area of the paralelepid A is equal to edge x multiplied by edge y), so, dA is equal dx y + x dy. See: But this is so much incovenient! The convenient would be dA = dx dy. Let's see now d²A... d²A = d dA = d²x y + dx dy + dx dy + x d²y Now dx dy appears! But, is not a convenient expression, because d²x y and x d²y appears too in the equation! How to solve this problem? We want that A = x y and dA = dx dy. Do you understand this conceptual problem of defition?