\begin{align*}\small x = \frac {-b \pm \sqrt {b^2 - 4ac}}{2a}\\ \end{align*}

\begin{align*}\small \sum\limits_{n = - \infty }^\infty {\left| {x(n)} \right|^2 } = \frac{1}{{2\pi }}\int\limits_{ - \pi }^\pi {\left| {X(e^{j\omega } )} \right|^2 d\omega } \end{align*}

\begin{align*}\small \sqrt{(x_1 - x_2)^2 + (y_1 - y_2)^2} \end{align*}

\begin{align*}\small \cos x = \sum\limits_{n = 0}^\infty {\frac{{\left( { - 1} \right)^n x^{2n} }}{{\left( {2n} \right)!}}} \end{align*}

\begin{align*}\small e = \mathop {\lim }\limits_{n \to \infty } \left( {1 + \frac{1}{n}} \right)^n \end{align*}

\begin{align*}\small P = 2\pi r \end{align*}