For T < Tc, the calculated isotherms oscillate, and each one passes through a minimum
followed by a maximum. These extrema converge as T→Tc and coincide at T = Tc; at
the critical point the curve has a flat inflexion (4). From the properties of curves, we
know that an inflexion of this type occurs when both the first and second derivatives
are zero. Hence, we can find the critical constants by calculating these derivatives and
setting them equal to zero