A mathematical formula is a mathematical expression using mathematical language and mathematical symbols, used to define a concept or relationship, mathematically. Examples of mathematical formulae include equations, logical formulae, and recursive functions. The symbols that may appear include variables and quanitifiers, as well as symbols specific to the field in which the concept is expressed.
The accepted plurals of formula are formulae or formulas. There is no limit how long formulae may be.
Formulae can be very simple, such as:
or complicated, such as this formulation of the Axiom of Choice: