The equation for a straight line is y = mx + b (1) and the equation for a polynomial is y = \sum_{n=0}^{\infty} a_n x^n (2)
Equation 1 and eq. 2 are known to all first-year math students.
The Fourier series is a little more advanced: y = \frac{1}{2}a_0 + \sum_{n=1}^{\infty}a_n\cos(nx) + \sum_{n=1}^{\infty}b_n\cos(nx) (3)
Equations 1–3 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.: \pi = 3.141592653589793238462643 \ldots (4)