Even though x is one of the least-used letters in the English alphabet, it appears throughout American culture ' from Stan Lee's X-Men superheroes to 'The X-Files' TV series. The letter x often symbolizes something unknown, with an air of mystery that can be appealing ' just look at Elon Musk with SpaceX, Tesla's Model X, and now X as a new name for Twitter. You might be most familiar with x from math class. Many algebra problems use x as a variable, to stand in for an unknown quantity. But why is x the letter chosen for this role' When and where did this convention begin' There are a few different explanations that math enthusiasts have put forward ' some citing translation, others pointing to a more typographic origin. Each theory has some merit, but historians of mathematics, like me, know that it's difficult to say for sure how x got its role in modern algebra. Algebra today is a branch of math in which abstract symbols are manipulated, using arithmetic, to solve different kinds...
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