x the unknown

Anyone who has discovered the wonders of algebra has wondered, "why is x the letter always used to represent the unknown quantity one wishes to find?" Or perhaps not that many have wondered. At first one is too busy learning the actual algebra to think about it and later on the x has been invoked so many times that it embeds in woodwork too deep to be dug out.

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But what is the origin actually? Is there something inherently mysterious about x perhaps? After all, it is the beginning of only very few words. Maybe it's the slanty, chic appearance. It "marks the spot", seeming to keep at the center of its intersecting lines an important secret. Probably one or more of these are the reasons, right?

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Actually, no, although, the reasons above may have helped to keep x popular in this use. But the actual beginning comes, like algebra (from the Arabic al-jabr, meaning, "the reduction"), from the Islamic world. The treatise of the philsopher Omar Khayyam (1048-1122) describes solving for shay, which is Arabic for "the thing". When this learning came to the Western world via scientific works in Islamic Spain, it was spelled "xay". For convenience this was gradually reduced to just x.
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