François Viète, a French mathematician, was the first to make systematic use of letters to refer to unknown. quantities. In his writings, vowels represented unknown quantities and consonants represented known quantities. Decades later, René Descartes, in his book La Géométrie (1637), used the first letters of the alphabet for known quantities and the last letters of the alphabet for unknown quantities. For example, the equation [latex]ax + by = c[/latex], introduced by Descartes, is still used today to describe the equation of a line. Viète introduced braces in 1593. François Viète (1540-1603)