Derivative of a quadratic form

Let $A\in {\mathcal{M}}_{n,n}(\mathbf{R})$ be an $n$ by $n$ real-valued matrix, and let $f:{\mathbf{R}}^n\to \mathbf{R}$ be defined by $f(x)=x^{\mathrm{T}}Ax$. On this page, we calculate the derivative of $f$.