Short answers, pulled from the story.
The French mathematician René Descartes assigned the letters x, y, and z to represent unknowns in the 17th century. He flipped the previous convention used by François Viète, which reserved vowels for unknowns and consonants for known values. This decision persists in classrooms and laboratories today.
René Descartes published the book La Géométrie in 1637. This publication transformed algebra from a rhetorical discipline into a symbolic one. The work introduced the convention of using x, y, and z for unknowns.
The choice of x to represent the unknown was likely influenced by the Arabic word for unknown, shay. This word was transliterated into Spanish as xay and eventually shortened to x. The linguistic journey moved from the Middle East to the pages of Descartes' La Géométrie.
Euclid wrote his work Elements around 300 BC. In this text, he described algebraic relationships through geometric figures rather than equations. He stated that if two straight lines are cut into segments, the rectangle contained by the two lines is equal to the sum of the rectangles contained by the uncut line and each segment.
The mathematician Karl Weierstrass formalized the notion of a limit in the 19th century. He introduced the epsilon-delta definition, which removed the need for variables to actually vary or move. This shift treated variables as static symbols that could be substituted with any element from a given set.
A parameter is a quantity that is part of the input of a problem and remains constant throughout the solution, while an unknown is a variable that must be determined. A general cubic equation contains five variables, consisting of four parameters and one unknown. The parameters are given numbers that define the shape of the curve, while the unknown is the value that satisfies the equation.