Solution
= Solution
Treat $R$ and $a$ as constants and use the <total differential> in the independent variables $n,V,T$:
$$
\boxed{dp=\left(\frac{RT}{V}-\frac{2na}{V^2}\right)dn
+\left(-\frac{nRT}{V^2}+\frac{2n^2a}{V^3}\right)dV
+\frac{nR}{V}\,dT}.
$$
Each coefficient differentiates with respect to its displayed variable while holding the other two fixed; the formula applies where $V\ne0$.