\documentclass[a4paper]{article} \begin{document} Stokes' Theorem: \[ \int \int (\vec{\nabla} \times \vec{A}) \vec{df} = \oint \vec{A} \vec{dl} \] Euler-Lagrange Equations: \[ \frac{d}{dx} \left(\frac{\partial F}{\partial y'_i} \right) - \frac{\partial F}{\partial y_i} = 0 \] \end{document}