# 4.8 | Differentials

Definition of Differentials
Let $$y = f(x)$$ represent a function that is differentiable on an open interval containing $$x$$. The differential of $$x$$ (denoted $$dx$$) is any nonzero real number. The differential of $$y$$ (denoted $$dy$$) is
$$dy = f'(x) \; dx$$