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In my textbook I read this example:

A gas in an isolated system expands after pulling out a separating plate, so its volume increases and there is no work or heat exchange.

the entropy of if the ideal gas is

$$\Delta S = n R \ln \frac {V_1} {V_2}$$

and the second law of thermodynamics is

$$\Delta S = \int \frac {\delta Q} T = 0$$

But I didn't understand which of the two formula is the true one?