"For nonzero cosmological constant ?, we show that the barotropic FRW cosmologies as worked out in the comoving time lead in the radiation-dominated case to scale factors of identical form as for the Chiellini dissipative scale factors in conformal time obtained recently by us in Phys. Lett. A 379, 882 (2015). This is due to the Ermakov equation which is obtained in this case. For zero cosmological constant, several textbook solutions are easily obtained as particular cases of our formulas and hypergeometric solutions are also reminded."