Unknown is the value to be determined when solving the equation. In ancient China and China, unknowns were not represented by symbols, but equations were solved by calculation. In the "Tianyuan Shu" of Song and Yuan Dynasties, the unknown is represented by "establishing Tianyuan", and a yuan word is written next to the corresponding coefficient as a symbol. In the Yuan Dynasty, Zhu Shijie (about13rd century) expressed four unknowns in terms of heaven, earth, people and things, and established the theory of higher-order quaternary equations. At present, the names of elements in the problem of elimination in mathematics are also derived from Diophantine diagrams (about 246-330) in western ancient Greece, which indicate unknowns with letters, but later progress is slow. In the past, different unknowns were represented by the same symbol, which was easy to be confused. So in 1559, the French mathematician Bert (1485 to 1492- 1560 to 1572) began to express different unknowns, and in1572, 1637, Descartes (1596- 1650) began to express the unknowns of positive numbers with x, y and z in geometry. It was not until 1657 that John Harder began to use letters to represent the unknowns of positive and negative numbers.