The ability to distinguish between exact and inexact differentials is an important part of solving first-order differential equations of the form Adx + Bdy = 0, where A(x,y) [not equal to] 0 and B(x,y) [not equal to] 0 are functions of x and y However, although most undergraduate textbooks motivate the necessary condition for exactness, i.e. the requirement that aA/ay = aB/ax, simple proofs that this condition is sufficient are not always given. Here we demonstrate that the condition is sufficient using a novel proof by contradiction. Our approach is compared to other proofs existing in the literature, and considered in the context of personalized learning.