\[V_{1} - начальный\ объем;\]
\[V_{2} - конечный\ объем;\]
\[\text{v\ }моль - количество\ газа.\]
\[1)\ Уравнение\ идеального\ газа:\]
\[PV = vRT;\]
\[P(V) = \frac{\text{vRT}}{V}.\]
\[2)\ A = \int_{V_{1}}^{V_{2}}{P(V)\text{\ dx}} =\]
\[= \int_{V_{1}}^{V_{2}}{\frac{\text{vRT}}{V}\text{dx}} = \left. \ \text{vRT}\ln|V| \right|_{V_{1}}^{V_{2}} =\]
\[= vRT \bullet \ln V_{2} - vRT \bullet \ln V_{1} =\]
\[= vRT \bullet \ln\frac{V_{2}}{V_{1}}.\]
\[Ответ:\ \ vRT \bullet \ln\frac{V_{2}}{V_{1}}.\]