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In [[mathematics]], when a letter is used to represent a number or set of numbers it is called an '''unknown'''.  This letter usually has a very specific range of values, unlike a [[variable]] which can be changed arbitrarily to alter the value of a [[function]] which acts on the variable.<ref>http://www.icoachmath.com/Sitemap/Unknown.html</ref>

In [[mathematics]], when a letter is used to represent a number or set of numbers it is called an '''unknown'''.  This letter usually has a very specific range of values, unlike a [[variable]] which can be changed arbitrarily to alter the value of a [[function]] which acts on the variable.<ref>http://www.icoachmath.com/Sitemap/Unknown.html</ref>