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We give an explicit formula for the symbol of a function of an operator. Given a pseudo-differential operator $\widehat{A}$ on $L^2\left({ℝ}^N\right)$ with symbol $A\in {𝒞}^{\infty }\left(T^{*}{ℝ}^N\right)$ and a smooth function $f$, we obtain the symbol of $f\left(\widehat{A}\right)$ in terms of $A$. As an application, Bohr-Sommerfeld quantization rules are explicitly calculated at order 4 in $\hslash$.