The diagram shows the pressure and volume of an ideal gas during one cycle of an engine, figure one as the gas proceeds from state one to state to. It is heated at constant pressure. It is then cold at constant volume until it reaches state three the gases, then cold at constant pressure to stay for finally the gas is heated at constant volume until it returns to state one when the gases in one it’s temperature is T1 the temperature T3 of the gas, when it is in state three expressed T3, in terms of T1.