  In this paper, we first review the equal area laws and Clapeyron equations in
the extended phase space of the charged anti-de Sitter black holes. With
different fixed parameters, the Maxwell's equal area law holds not only in the
pressure-thermodynamic volume oscillatory line, but also in the charge-electric
potential and temperature-entropy oscillatory lines. The conventional Clapeyron
equation is generalized and two extra equations are found. Moreover, we show
that the coexistence curve of the small and large charged black holes is charge
independent in the reduced parameter space for any dimension of spacetime. The
highly accurate fitting formula for the coexistence curve is also presented.
Using this fitting formula of the coexistence curve, we find that the Clapeyron
equations are highly consistent with the calculated values. The fitting formula
is also very useful for further study on the thermodynamic property of the
system varying along the coexistence curve.
