# Why is the Unknown in Mathematics Called X?

**Why is the Unknown in Mathematics Called X?**

Although it has not been conclusively proven, the reason why the unknown in mathematics is called x is because Europeans could not find a letter in the Latin alphabet that would make the Arabic “thing” sound, so they chose the letter closest to the Greek alphabet “thing” sound.

Today, we all understand and use mathematical symbols such as “2+3 = 5” or “x-3 = 8” when performing mathematical operations. These symbols and signs mean the language of mathematics, and all of these symbols are also called mathematical notation. Today, when you want to express a mathematical operation, you use mathematical notation. But these symbols, like languages, accumulate and evolve over time.

In the past, there was no uniform worldwide mathematical notation. For this reason, mathematical operations were written not as “2+3=5”, but with verbal expressions such as “If you add two and three, you get five”. There were even those who took this work further and put mathematical formulas into poems. e.g; Today, the solution formula of the cubic equation is expressed in mathematical notation as in the picture below:

For example, Niccolo Tartaglia, a mathematician in the Middle Ages, where we do not yet have universal mathematical notation, wrote the solution formula he found for the cubic equation in the form of a poem, as in the picture:

**X expression in Mathematics**

In 820, when Khwarazmi was in the golden age of Islam, he wrote a book titled “al’kitab’ul-muhtasar fi hisab’il cebri wa’l-mukabele, that is, a summary book on algebra and the calculation of equations”. This book is the first algebra book in history. What we call algebra can be summarized as “doing mathematical operations by expressing numbers with unknown values with symbols”.

While writing this book, Khwarizmi had come up with the idea of algebra, but the symbols we use today like 1,2,3,4 did not exist. Just as we can say one, two, three, four instead of 1,2,3,4 in English, all operations were carried out verbally.

*An example from Khwarizmi’s book:*

In the algebraic chapter of this book, he poses a similar theorem when performing algebraic operations: Let a “thing” be equal to the sum of the numbers two and three. then this “thing” is equal to five.

We draw your attention, the word we call “thing” in English is actually of Arabic origin and the Arabs pronounce the word “be” as “thing”. Just as people today do not use words in their formal form in daily life, then this was the case. In other words, the person who reads the sentence “Let something equal five”, written by Khwarizmi, reads it as “Let something be equal to five”.

**See Also: What is Torque Converter? How does Torque Converter work?**

This book is very famous at that time, it goes from North Africa to Spain. European mathematicians who read the book here decided to translate the book into Latin languages, but because they could not find a letter to pronounce “shi” in Latin languages, they decided to use the letter “ki” in the Greek alphabet with the script “because we did not call it shi”.

Since the Europeans decided to use x instead of shi, they wrote the Khwarizmi formula as “let it be one or five” instead of “let it be one or five”.

At that time, the letter “ki” was written as an x in the Greek alphabet. In other words, when Europeans write “let it be”, they write “let it be x”. We got our mathematical notation from Europeans in modern times, as most of the modern universal mathematical notation spread to the European continent in the future. Today we call the unknown x because Europeans use the letter x as the symbol for the word “thing” in their mathematical notation.

So in short; The formula written as “what is x if x = 2+3” is “if thing = 2+3 what is this thing?” means.

**Different Theory for X Expression in Mathematics**

There are also those who think that this formula comes from the groundbreaking work “Discours de la méthode pour bien conduire sa raison et chercher la verité dans les sciences” published in 1637 by the French scholar René Descartes. The baroque title in Turkish is “Speech on Method”. In this work, Descartes deals with philosophy as well as mathematics. Most people are familiar with the famous phrase “I think, therefore I am” used in this book, and the equally well-known “Cartesian coordinate system” probably named after Descartes. He also discussed the connection between the thing and x in this book.