(b) (5 points) In equilibrium, heat Q can be written as Q=ST, S is entropy; the work W=PV if only pressure P is applied to the system. V is the volume of the system. If we allow the system to change infinitely slowly (so to maintain the equilibrium), so dU=dQ-dW. From U=Q-W and dU=dQ-dw. derive Gibbs-Duhem relation