and use them to express 481 = (13)(37) as the sum of two squares in two different ways.
These identities were given later, in 1202, by Fibonacci in his Liber abaci. They show that the product of two numbers each expressible as the sum of two squares is also expressible as the sum of two squares. It can be shown that these identities include the addition formulas for the sine and cosine. The identities later became the germ of the Gaussian theory of arithmetical quadratic forms and of certain developments in modern algebra.