$\mathrm{PT}^{-3}=\text { constant }$
$\because \frac{\mathrm{PV}}{\mathrm{T}}=\mathrm{nR}=\text { constant from ideal gas equation }$
$(\mathrm{P})(\mathrm{PV})^{-3}=\text { constant }$
$\mathrm{P}^{-2} \mathrm{~V}^{-3}=\text { cosntant } \quad . .(1)$
$\because \text { Process equation for adiabatic process is }$
$\mathrm{PV}^y=\text { constant } \quad . .(2)$
$\text { Comparing equation (1) and (2) }$
$\frac{\mathrm{C}_{\mathrm{p}}}{\mathrm{C}_{\mathrm{V}}}=\mathrm{y}=\frac{3}{2}$



