# Definition

Nonlinear equations are those that cannot be written in the form
$\vv{A}\vv{x} = \vv{b}$. Such equations come up in many places:

- Equations of state $P(T, V)$
- Second-order reactions $2\,{\rm A} \to {\rm B}$, $r_{\rm A} = -k c_{\rm A}^2$
- Optimization/process design of full systems