\begin{matrix} x = \frac{- b \pm \sqrt{b^{2} - 4 a c}}{2 a} \\ A = (\begin{matrix} \cos θ & - \sin θ \\ \sin θ & \cos θ \end{matrix}) \\ \sum_{n = 1}^{\infty} \frac{1}{n^{2}} = \frac{π^{2}}{6}, \int_{0}^{1} e^{x} ⅆ x = e - 1 \end{matrix}