  We determine the equation of state of QCD at finite chemical potential, to
order $(\mu_B/T)^6$, for a system of 2+1 quark flavors. The simulations are
performed at the physical mass for the light and strange quarks on several
lattice spacings; the results are continuum extrapolated using lattices of up
to $N_t=16$ temporal resolution. The QCD pressure and interaction measure are
calculated along the isentropic trajectories in the $(T,~\mu_B)$ plane
corresponding to the RHIC Beam Energy Scan collision energies. Their behavior
is determined through analytic continuation from imaginary chemical potentials
of the baryonic density. We also determine the Taylor expansion coefficients
around $\mu_B=0$ from the simulations at imaginary chemical potentials.
Strangeness neutrality and charge conservation are imposed, to match the
experimental conditions.
