An unknown quantity

This lesson asks you to describe how a variable is used and to use one in problems. The museum offers a record on algebra, the branch of mathematics that solves problems by positing an unknown. Choosing a letter or symbol and using it in your own problems is the class's work. ¹

Make up a problem with a number you do not know yet. Give the unknown a letter. What does the letter stand for in your problem?

Al-Khwarizmi wrote a book about finding a number you do not know. He used words, not symbols, for everything: the unknown was called a “thing”, and its square a mal ². The science of algebra takes its name from the word al-jabr, which he used as the title of his book ³.

Al-Khwarizmi wrote in words, not symbols, and called the unknown number a “thing”. In your problem about marbles, which would help a friend more, the word “thing” or a letter, and why?

• Algebra (al-jabr): where the name of the science comes from.
• The Book of al-Jabr and al-Muqabala: the book, and how it writes everything in words.
• al-Khwarizmi: the scholar who wrote it.

Read the record on algebra. Write a short problem about an unknown number of marbles, give the unknown a letter, and explain to a partner what it stands for.