Consider the curves \( y = f(x) \) and \( x = g(y) \), and let \( P(x,y) \) be a common point of these curves.
If at \( P \), on the curve \( y = f(x) \),
\[
\frac{dy}{dx} = Q(x),
\]
and at the same point \( P \) on the curve \( x = g(y) \),
\[
\frac{dx}{dy} = -Q(x),
\]
then: