$\mathrm{P}=0.40 \mathrm{V}^{-2.5}$
$\frac{\mathrm{dP}}{\mathrm{dV}}=0.40\left[-2.5 \mathrm{V}^{-3.5}\right]$
at $\quad V=1$
$\left(\frac{\mathrm{dP}}{\mathrm{dV}}\right)_{\mathrm{V}=1}=-1$

$(A)$ Internal energies at $\mathrm{A}$ and $\mathrm{B}$ are the same
$(B)$ Work done by the gas in process $\mathrm{AB}$ is $\mathrm{P}_0 \mathrm{~V}_0 \ln 4$
$(C)$ Pressure at $C$ is $\frac{P_0}{4}$
$(D)$ Temperature at $\mathrm{C}$ is $\frac{\mathrm{T}_0}{4}$

