Why does the letter “x” represent the unknown?

Probably one of the most common variables you will ever run across in mathematics is “x.” You have undoubtedly seen it countless times, in math problems asking you to simply “solve for x,” or as one of several different letters in a multivariable equation. Using the letter x to denote unknowns has even gone beyond mathematics – just look at The X Files on TV, where x indicates mysterious and unknown FBI cases, or even to the historical example of naming the x-ray, where x indicated a radiation coming from an unknown source. By now, you probably take for granted that x is the go-to letter when you want to represent some unknown quantity. While any letter can be used, I’m sure that you will agree that x is by far the most common one used. Have you ever stopped to wonder why? Why is x the unknown?

Where did this convention come from to let some seemingly random letter become the single one letter most often used to represent the unknown. Interestingly, it has its roots in language. Specifically, it is related to phonetics used in translating from Arabic, and the lack of corresponding phonetic sounds in Spanish.

It is very interesting, and I will let Terry Moore describe in far more detail in the **TED video** clip. It’s only a few minutes long, and it builds up to a nice tongue-in-cheek joke at the very end – especially if you have any Spanish friends!

Give this video a watch, and find out why x is the unknown!

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