$6\mathrm{.}11$ Combining Eqs. $6\mathrm{.}2-9$ and $6\mathrm{.}2-12$ we have

$P=kT(\frac{delln}{\text{d}\text{e}\text{l}\text{V}}{)}_{,T}=\frac{kT}{V}ln$

Mathematically explain how this is possible.