"""The use of z, y, x . . . to represent unknowns is due to René Descartes, in his La géometrie (1637). Without comment, he introduces the use of the first letters of the alphabet to signify known quantities and the use of the last letters to signify unknown quantities."""

...And there is also no evidence that it came from a strange transliteration of an arabic word, like that TED talk says.
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