A heat engine designed for operation in outer space is to be analyzed as if it was a 2-T Carnot device. While the high-temperature side of the engine can be sustained at Th in several ways, the only way to reject heat from the low-temp side of such an engine is by radiation heat transfer to outer space, in which case the rate of heat transfer is proportional to the produc of radiating area A and the fourth power of radiating temperature Tl^4. Show that, for a given power output and a given Th, the area of the radiator will be minimum when Tl/Th = 3/4.