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## Homework Statement

I'm having a little trouble knowing when to use the ideal gas equations for entropy vs just the ones like this: (T2/T1)=(p2/p1)^((k-1)/k).

I've noticed a pattern in the solutions for my homework( where you're finding isentropic efficiency of turbines and compressors) they tend to use the ideal gas equations for entropy to find the real values for the properties and then they use the ones like this:(T2/T1)=(p2/p1)^((k-1)/k) to find the 2s values.

Is my assumption correct? Sorry if its poorly worded, I'm still trying to figure this stuff out :)

If it is correct, can you please explain why? Thanks!