Do you know the most missing thing in the world is "X"? Either word, letter, humans, animals, or even thing. If you are to solve their problem, X is always the missing culprit. It looks like we are just obsessed with X that we name so many things, rate so many things, and even sign letters when we want to be anonymous, not also forgetting that Elon named his newly acquired company after the same letter although it looks like he likes X a lot from SpaceX to Tesla Model X. But then, depending on your source, the letter X is regarded as the third or fourth least used alphabet as it doesn't always come along well in most words and in words where it is found, it can be replaced with other letters but then Mathematics made it neccessary.
This started about 400 years ago when it began to show up in text books as a placeholder when looking for an unknown solution. But then, X didn't just appear in english language, it has been a letter for over 2000 years in different languages such as X being "Chi" in ancient Greek words, then in Latin X was regarded as "z". Before the beginning of solving for unknown, mathematics was basically of the known where questions like how many objects were present and so on, but then ancient Babylonians decided to start looking for unknown to solve agricultural and other problems and this time around, the unknown was in the question.
The questions were surrounded around plots of lands and the questions into the unknown led to the first equation we probably first heard, the quadratic equation. While the Babylonians were using X for their unknown, the ancient Greeks,Indians, Egyptians and Chinese didn't use X for their unknown although they were solving questions. These mathematicians wrote out their unknown problems in full sentences like Brahmagupta who was a mathematician in ancient India referred to the unknown as "YAVATTAVAT" meaning "Something", so you would understand what I meant by writing the unknown in full.
As time goes by, Persian Scholar Al-khwarizni in the Ninth century started to work out solutions to different algebraic problems and gave the word "Algebra" where he explained the need of adding equal value to an equation in other to solve for the unknown. For Instance ~ X - 5 = 25. He decided that we add equal value of the number to the get the equation solved, so we have 5 - X - 5 = 25 + 5 which then becomes X = 30.
The way Al-khwarizni solved algebra with algorithm made it easy for anyone to solve most mathematical problems they came across but then he didn't use X as the unknown like I wrote above, instead he wrote it in full but then mathematicians began to look at changing those words with letters such as A, B, and so on. This continued even in the 1500s with French mathematicians like Francois Viete writing mathematical problems in different letters but he basically used vowels but still added words.
As this continued, different mathematicians used different symbols along with words, so it became difficult for one mathematician to understand the others mathematician's math. Soon, french mathematicians began to use an unknown that was popular in the mathematical world which was X but how it became popular is still unknown although there are several theories to this, such as one being a Spanish misinterpretation of Al-khwarizni mathematical problem, Another theory traces the use of X to French Mathematician Rene Descartes who used graph to solve to problems to give different solutions to different unknowns. He used letter from the beginning of the alphabet to represent known values and letters from the end of the alphabet to represent unknown values but when he was done writing his book, he was using X to represent the unknown more than any other letter from the end of the alphabet.
While it has been very difficult to place a solid historical background to where X began from, it is no doubt that since the days of French Mathematician Rene Descartes, X has become a thing in Mathematics and other mathematicians who built on his work continued to use X. Since we have been able to unify the unknown, it has been easy to use algebra to solve a lot of problems including real life problems.
Reference
https://academic.oup.com/philmat/article/14/3/287/1462575?login=false
https://www.sciencedirect.com/science/article/pii/S031508600600022X
https://jontalle.web.engr.illinois.edu/uploads/298/HistoryMath-Burton.85.pdf
http://math.fau.edu/yiu/PSRM2015
https://jontallen.ece.illinois.edu/uploads/537.F18/Papers/MathematicsandItsHistory-johnStillwell.pdf
https://bigthink.com/hard-science/quadratic-formula-history/
https://theconversation.com/x-marks-the-unknown-in-algebra-but-xs-origins-are-a-math-mystery-210440
https://scholarship.claremont.edu/cgi/viewcontent.cgi
https://www.jstor.org/stable/2299605
https://link.springer.com/book/10.1007/978-3-642-61779-9
https://link.springer.com/book/10.1007/978-3-642-51599-6
https://www.jstor.org/stable/27963851
https://www.jstor.org/stable/2972623
https://ia802209.us.archive.org/9/items/taming-the-unknown-a-history-of-algebra
https://www.ams.org/journals/bull/2015-52-04/S0273-0979-2015-01491-6/S0273-0979-2015-01491-6.pdf
https://books.google.com/books?id=I-jwDQAAQBAJ&pg=PA73&lpg=PA73
https://ia902207.us.archive.org/5/items/geometryofrene00desc/geometryofrene00desc.pdf
So the origin of X can be traced but cannot be confirmed. X has really become a big deal in our world. I remember in high school, there was a time I used Q as my unknown and my teacher failed me. He made it clear to me that X is the major unknown and in rare cases when X is known, Y becomes the unknown. It became more serious with calculus but all the same, I still wonder till date why he didn't just take Q as the unknown instead of making a whole scene out of ordinary letters.
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