How did symbols for unknowns and knowns in algebraic equations develop?
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In 1591, François Viète was the first to write and solve general algebraic equations by introducing the systematic use of letters as algebraic symbols. He used vowels (a, e, i, o, u) for the unknowns and consonants (the rest of the alphabet) for the coefficients (or knowns).
But it was René Descartes who introduced a new way of using letters in the alphabet in his work, La Gèometrie. He used the letters at the end of the alphabet (x, y, …) for unknowns and beginning of the alphabet (a, b, …) for knowns (in many instances, these letters are italicized). This standard is still in algebraic use today.