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# Formula

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**. Here, if x is, for example, 1, then y = x so y = 1. Possibly the most famous formula is **e = m × c ^{2}**. Albert Einstein created this. In it, e is energy, m is mass and c is the speed of light. Thus, energy = mass × (multiplied by) the speed of light

^{2}(squared). The multiplication sign is usually left out, giving

**e = mc**.

^{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.