Algebraic Variables
1646
The work of French mathematician François Viète (1540-1603) marked the transition from medieval mathematics to modern mathematics. He is known as the father of algebra. He introduced the systematic representation of letters for unknown numbers. He also recommended uniform methods of solving equations.

Illustration from:
Francisci Viet Opera Mathematica, In Unum Volumen Congesta, Ac Regonita, 1646. It was published in Leiden, ex officina Bonaventurae & Abrahami Elzeviriorum (from the offices of Bonadventure and Abraham Elsevier).