Short answers, pulled from the story.
The Moscow Mathematical Papyrus from around 1500 BC contains problems that ancient Egyptian scribes called Aha problems. These puzzles required finding an unknown quantity, which the text referred to as aha or stack.
François Viète introduced a system at the end of the 16th century where consonants represented known values and vowels represented unknowns. René Descartes changed this convention in 1637 by using letters near the start of the alphabet for knowns and letters near the end for unknowns.
Karl Weierstrass addressed paradoxes in calculus during the second half of the 19th century by replacing intuitive limits with formal definitions involving epsilon and delta. This static formulation meant none of the five variables were considered as varying within the definition itself.
Letters at the beginning of the alphabet such as a, b, c denote constants while later letters like u, v, w, x, y, z represent variables. Greek letters including alpha, beta, gamma often follow similar patterns for parameters and coefficients.
In calculus, a dependent variable represents the value of a function whose possible values depend on another independent variable. An independent variable is one that does not depend on other variables in the given context.