  We exactly solve the relaxation-time approximation Boltzmann equation for a
system which is transversely homogeneous and undergoing boost-invariant
longitudinal expansion. We compare the resulting exact numerical solution with
approximate solutions available in the anisotropic hydrodynamics and second
order viscous hydrodynamics frameworks. In all cases studied, we find that the
anisotropic hydrodynamics framework is a better approximation to the exact
solution than traditional viscous hydrodynamical approaches.
