Trairāśika

Summary

Trairāśika is the Sanskrit term used by Indian astronomers and mathematicians of the pre-modern era to denote what is known as the "rule of three" in elementary mathematics and algebra. In the contemporary mathematical literature, the term "rule of three" refers to the principle of cross-multiplication which states that if $\tfrac{a}{b}=\tfrac{c}{d}$ then $ad=bc$ or $a=\tfrac{bc}{d}$. The antiquity of the term trairāśika is attested by its presence in the Bakhshali manuscript, a document believed to have been composed in the early centuries of the Common Era.