A heat engine operating in outer space may be assumed equivalent to a Carnot engine operating between reservoirs at temperatures TH and Tc The only way heat can be discarded from the engine is by radiation, the rate of which is given (approximately) by:
Qc = kA(Tc^4)
Prove that, for fixed power output and for fixed temperature TH, the radiator area A is a minimum when the temperature ratio Tc/TH is 0.75