Reading Grafakos' book "Classical Fourier Analysis", I got stuck in the Exercise $2.3.5.$ (a) which states that if $f ∈ \mathscr{S}(\mathbb{R}^n)$ then $$(\tau^{−he_j} f − f )/h\rightarrow \partial_j f\qquad \text{ in } \mathscr{S}(\mathbb{R}^n)\text{ as } h\rightarrow 0,$$ where $\tau^{−he_j} f(x) := f(x+he_j)$.
Could anyone give me some hints? I was thinking about a mean value argument. Thank you.