1585, the French mathematician Wei Dachuang used capital vowel letters A, E, I and O to represent unknowns, and consonant letters B, G and D to represent ordinary letters. In the17th century, the mathematician Descartes improved the Vedic alphabet. He uses the first letter of the alphabet to represent the known number, and the last three letters X, Y and Z represent the unknown number. Since then, X, Y and Z have been widely used.