The equation for a straight line is \begin{equation} y = mx + b \label{eq:line}\end{equation} and the equation for a polynomial is \begin{equation} y = \sum_{n=0}^{\infty} a_n x^n \label{eq:polynomial}\end{equation} Equation \ref{eq:line} and eq. \ref{eq:polynomial} are known to all first-year math students. The Fourier series is a little more advanced: \begin{equation} y = \frac{1}{2}a_0 + \sum_{n=1}^{\infty}a_n\cos(nx) + \sum_{n=1}^{\infty}b_n\cos(nx) \label{eq:fourier}\end{equation} Equations \ref{eq:line}--\ref{eq:fourier} are used throughout science and engineering. Equations can be left unnumbered if we do not need to refer to them: \[ y = A e^{-\gamma t}\cos(2\pi f t) \] It is also possible to number equations generically without planning to refer to them; e.g.: \begin{equation} \pi = 3.141592653589793238462643 \ldots \end{equation}