We have always been used to using X to represent the unknown, which seems to be true. That's what the teacher taught us, and that's what the book says, so we accepted it. Few people will ask why things are too obvious. So what is the origin of the unknown represented by X? In fact, it is really not simple, which involves the spread and evolution of language and the history of algebra.

Algebra first originated in the Middle East and then spread all over the world. Therefore, there is a hypothesis in the academic circles recently that when contemporary numbers were spread to Spain, the Spanish could not pronounce some Arabic pronunciations well. In Arabic, "unknown things" are represented by the word "al-shalan", which frequently appears in early mathematical works. But there is no pronunciation like "sh" in Spanish, only "ck", which is the pronunciation of the letter "X" in the ancient Greek alphabet.

Later translated into Latin chi(x), and then evolved into a more common Latin X, somewhat similar to the usual abbreviation Xmas for Christmas. At present, this hypothesis is still controversial in academic circles, so it is inconclusive.

There is another saying in Webster's Dictionary that when Arabic words "thing" and "shei" are translated into Greek, they are expressed as "xei" and then abbreviated as X. xenos is an unknown meaning in Greek and also starts with X, so X may be the abbreviation of Xenos. At present, these two abbreviation theories have not found direct arguments to support them.

There is also a more detailed statement that the use of X for unknown originated from mathematician Descartes, but it is impossible to verify whether he invented it himself or borrowed it from others. But the works recorded so far show that he is the first person to use "X" to represent the unknown. In Geometry, he began to use lowercase letters A, B and C to represent known quantities, and lowercase letters X, Y and Z to represent unknown quantities.

Whether Descartes initiated it or not, with his academic influence, he widely popularized the usage of this symbol mark, which is still in use today. He is also a leading hero.

Since x, y, z Y and z can all represent unknowns in Descartes' works, why does x become the default symbol of unknowns 1? There is an interesting speculation, perhaps because the X letter appears least in his book Geometry, so there are a large number of X letter blocks, which are only used to sort the unknowns.

This has a certain joke element, because he has already started to use it in manuscripts before geometry, and there is no rule in using X, Y and Z in the early days. He even used x, y and z to represent known quantities.

Perhaps in the end, in the process of slow evolution of mathematics, everyone formed a conventional law, using X to represent the unknown. This method of symbol replacement is simple and clear, and it greatly simplifies writing, so it is still in use today.