Why "X" is Used to Express the Unknown?

For hundreds of years, "X" has been the symbol of the unknown quantity in mathematical equations. So who started this practice? There are several theories as to why it is called "X". Let's examine them together and decide for yourself the most logical reason.
algebra; He was born in the middle east, in the golden age of medieval Islamic civilization (between 750 and 1258 AD), and was given its first form by Mohammed al-harezmi. Later, the book "al-jabr wal-mukabela" (later turned into an algebra name) was published in the 9th century. In these magnificent days, the Muslim administration and culture expanded to the Iberian Peninsula with the encouragement of the scholars with scholarships in science and mathematics.

So what does this have to do with the letter "x" in math? In a recent ted talk (see ted talks), the founder of radius foundation terry moore attributes the use of x to spanish academics' inability to translate some arabic sounds. including shin sound. According to moore, the word "al-shalan" in arabic, which means "unknown thing", has always appeared in the first studies in mathematics. for example, "three unknowns are equal to 15, so the unknowns are 5." You may encounter sentences like the first math studies.

Meanwhile, you can watch terry moore's ted speech I mentioned above at:

however, since the Spanish academics did not have a sound corresponding to the letters "sh" in the word shin, they went to use the "ck" sound, which is written in chi symbol (big x in greek) in classical greek. Moore continues his theory as follows; The "unknown thing" characterized by chi (x) in greek was later replaced by x in the latin alphabet. this usage is also seen in the use of xmas, which means christmas. It is similar to the practices made by religious scholars using the Greek letter chi (x) as a shortcut for "Christ."

The main problem in Moore's explanation is that there are no direct documents and evidence to support him. To speak more speculatively, those who translate these mathematics works do not care about phonetics, they care about the meaning of words. that is, whether they were "sh" or not, they thought it would be irrelevant. Although there is no direct evidence for this theory of moore, it remains the main theory very quickly, even among many scholars. (if you do a quick google search, you can find many foreign professors researching this theory in math)

In the 1909-1916 edition of the webster dictionary (now online version: webster's online dictionary) introduces another theory, among other theories. (a friend of ours wrote this theory here in 2011, to read: # 21814935.) This word, which means "thing" in Turkish, which corresponds to "shei" in Arabic, becomes "xei" in Greek, and then evolves to "x". d. ali khounsary also states that "xenos", which means "unknown" in greek, starts with x and may have been born from there. but again, it is useful to underline, as in other theories, there is no evidence or document to support this theory.

Let's come to a documented theory. Although the great philosopher and mathematician rene descartes does not find it and begins to see it and use it from someone else, it is shown as the architect of this use by oxford english dictionary and by the author of a history of mathematical notations florian cajori. at least descartes is important in spreading this use to large audiences, even if he did not find it himself.

In particular, his masterpiece, La Geometrie (1637), descartes symbolized the letters at the beginning of the alphabet (eg a, b, c) for known quantities, and the letters at the end of the alphabet (eg z, y, x) for unknown quantities.

but why? why x rather than z or y? still nobody knows. Speaking speculatively, x stands out because it is more popular when writing works than the other letter. one story even tells that the secretary who wrote the works of descartes was the one who suggested using x as unknown. because the use of x was less. This story is true or false or unknown, descartes used x in 1629 in some manuscripts that he wrote long before la geometrie. At that time, he used x, y, and z for the use of x, which was not so strict, even for known values.
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