Why is the word 'x' used for unknown symbols in mathematics?

For thousands of years, the "x" character has been used to indicate unknowns in mathematical functions. So where does this character come from and when does it start to be used? One theory accepted by scholars is due to language differences in the translation of Arabic-origin mathematical documents that gave birth to the "x" . Later, this principle continued to be popularized by Descartes mathematicians and became the common standard as it is today. So what happened to the story? Please find out.

Hypothesis: There is no corresponding sound

Algebra was born in the Middle East during the golden age of Muslim civilization (the Middle Ages from 750 to 1258 after CN) and the first forms were compiled into mathematical works in the 9th century. During this golden period, Islamic laws and civilizations were spread to the Iberian peninsula (now the Portuguese and Spanish territories, etc.) Here, Muslims arrested First teaches science subjects and including Mathematics.

An Arabic math material from Islamic civilization

So what does that have to do with the "x" in mathematics? According to some researchers, the "x" was born because Spanish scholars could not translate some sounds from Arabic. Accordingly, the word "unknown" in Arabic is "al-shalan". This is the term most used in the first math materials. Because in Spanish there is no sound corresponding to "sh" , the Spanish used "sk" instead. This is the ancient Greek sound and is represented by the letter X (the "chi " character).

Scientists hypothesize that after that, the X continues to translate into Latin and is replaced by the more popular x character. This is similar to the origin of the word Xmas, which is used by scholars to use the X (chi) in Greek instead of the word "Christ" .

However, the above explanations are based on hypothesis and speculation without concrete evidence. Moreover, translators of mathematical works will often not focus on pronunciation but only focus on conveying the meaning of words. Therefore, whether or not the "sh" sound is related to the "x" . However, many scholars including mathematicians still accept this argument.

In Webster dictionary version 1909-1916 and some other dictionaries, also use the same hypothesis to explain the origin of the "x" in mathematics. Although in Arabic, the singular "things" and "shei" have been translated into Latin as "xei" and then shortened to "x" . Some ideas also suggest that in Greek, hidden letters are written as "xenos" , beginning with an x, so abbreviations may also originate from here. However, it is also an unfounded argument.

Random choice of Descartes mathematician?

René Descartes (1596-1650), the famous author of the mathematical work La Géométrie, used the word x as an unknown and widely applied to this day

In the next era, the letter "x" continues to receive indirect support from the famous philosopher and mathematician René Descartes (1596-1650). Although Descartes did not directly regulate, in his writings and most famously La Géométrie (published in 1637), he used the letters at the top of the table (such as a, b, c, .) refers to known values ​​and the last letter of the table (such as x, y, z, .) to indicate unknown values ​​(unknowns).

An edition of La Géométrie's work by Descartes

Here you will ask, why is y, z not popular with unknown "x" ? No one knows that. One story is that it was because of Descartes's book La Géométrie who suggested that the "x" was the least used and that was the letter in which he had the most number of engravings. The story has not yet had a valid basis but in handwritten documents before La Géométrie was born, Descartes used "x" as an unknown. At the same time, Descartes was not too rigid, he used all three characters x, y, z to represent both unknown and known values. This further leads people to doubt the accuracy of the hypothesis "there is no sound when translating the Arabic word".

Therefore, it is possible that Descartes simply chose the letters to be most convenient to him . Anyway, one thing is certain that after the release of La Géométrie, the use of letters a, b, c to know the index and x, y, z to hide only became a practice and accepted to this day.

Reference: Gizmodo, Muslimheritage, Exzuberant, Wiki