Dealing with isothermal lines. How would you go about proving if the two lines are parallel if no values are given for the Pressure or the Volume? This is what I've got so far: Q = -W dU = 0, where U is internal energy. P1V1 = nRT P2V2 = nRT P1V1=P2V2 Or: P1/P2 = nRT1 / nRT2 The problem is I don't know how to start showing that the lines don't cross, i.e. the slopes are the same. Any help is greatly appreciated. Thank you.