FirstPkg
Here's an equation:
\[\frac{n!}{k!(n - k)!} = \binom{n}{k}\]
This is the binomial coefficient.
To write a system of equations, use the aligned environment:
\[\begin{aligned}
\nabla\cdot\mathbf{E} &= 4 \pi \rho \\
\nabla\cdot\mathbf{B} &= 0 \\
\nabla\times\mathbf{E} &= - \frac{1}{c} \frac{\partial\mathbf{B}}{\partial t} \\
\nabla\times\mathbf{B} &= - \frac{1}{c} \left(4 \pi \mathbf{J} + \frac{\partial\mathbf{E}}{\partial t} \right)
\end{aligned}\]
These are Maxwell's equations.