P1*V1=nRTP2*V2=nRTP1*V1=P2*V2P1=0.5MPaP2=0.1MPa(P2=0.2P1)V2=5V1因为恒外压（外压为0.1MPA）膨胀过程，做负功，求做功得用恒外压P2W=-P2*(V2-V1)= -P2*4V1 = -0.2P1*4V1 = -0.8P1*V1 = -0.8*nRT = -0.8*1*8.314*300 = -1995.36
