  Based on the method proposed in [ H. S. Zong, W. M. Sun, Phys. Rev. \textbf{D
78}, 054001 (2008)], we calculate the equation of state (EOS) of QCD at zero
temperature and finite quark chemical potential under the hard-dense-loop (HDL)
approximation. A comparison between the EOS under HDL approximation and the
cold, perturbative EOS of QCD proposed by Fraga, Pisarski and Schaffner-Bielich
is made. It is found that the pressure under HDL approximation is generally
smaller than the perturbative result. In addition, we also calculate the quark
number susceptibility (QNS) at finite temperature and finite chemical potential
under hard-thermal/dense-loop (HTL/HDL) approximation and compare our results
with the corresponding ones in the previous literature.
