Suppose we are given : PV = nRT, where n and R are constants. We are told to find the partial derivative dP/dV. Am I allowed to do this : P = nRT/V Then differentiate this w.r.t. to V. I disregarded the fact that V = 0 makes the RHS undefined. # This question came from Princeton Review's "Cracking the GRE Math Subject Test" page 160, qns 13. The solution given uses the above method but I do not understand why V = 0 is not taken into account.