 Quadratic-form ellipse area

For a two-dimensional symmetric positive-definite matrix $M$ and $R>0$, the region ${\mathbf{x}}^{T}M\mathbf{x}\le R$ is a filled ellipse of area $\pi R/\sqrt{\det M}$. By the spectral theorem for real symmetric matrices, an orthogonal change of coordinates gives positive eigenvalues ${\lambda }_1,{\lambda }_2$ and semiaxes $\sqrt{R/{\lambda }_1}$ and $\sqrt{R/{\lambda }_2}$. Their product and the area of an ellipse prove the formula. Translating the center of an ellipse leaves the area unchanged.