  The coupling parameter series expansion and the high temperature series
expansion in the ther- modynamic perturbation theory of fluids are shown to be
equivalent if the interaction potential is pairwise additive. As a consequence,
for the class of fluids with the potential having a hardcore repulsion, if the
hard-sphere fluid is chosen as reference system, the terms of coupling
parameter series expansion for radial distribution function, direct correlation
function and Helmholtz free energy follow a scaling law with temperature. The
scaling law is confirmed by application to square-well fluids.
