# Formula

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In mathematics or science a **formula** is a rule or statement written in algebraic symbols. The plural of formula can be written in two ways: formulae or formulas - the choice is based on personal preference.

Formulas use letters instead of words. A simple example of a formula is **y = x**. If x=1, the formula y=x would make the statement that x is the same thing as y, therefore, y=1. The most well known formula is the formula for energy, e = mc^{2 }, created by Albert Einstein. e represents energy, m represents mass and c is the speed of light. Thus, energy = mass × speed of light ^{2}.

The *formula* **e = mc ^{2}** is also an equation. Every equation is a formula, but not every formula is an equation. For instance, the formula x≡y (x "is defined as" y) is not an equation because no "=" sign is used.