Polytropic Process equation [Thermodynamics]

[nichmetsa]

TL;DR

(1/(1-n))*(p1v1-p2v2) into (n/(n-1))*(p1v1-p2v2)

Hello there,
So yesterday my thermodynamics professor did some black magic and transformed our beloved equation (1/(1-n))*(p1v1-p2v2) into (n/(n-1))*(p1v1-p2v2), but i didnt understand why he did it and how (he is too fast for my writing). Does anyone know how he did it, are there any restrictions to this formula? Just for context he was doing a introduction to the Rankine Cycle.

 

[Chestermiller]

The Rankine cycle is a continuous flow process involving a turbine (open system), rather than a bath process involving a piston and cylinder (closed system). In an ideal adiabatic open system process, the work is equal to the change in enthalpy, compared to a closed system, where the work is equal to the change in internal energy.