Question: prove that sqrt 5 is irrational?
Answer: 1. This irrationality proof for the square root of 5 uses Fermat's method of infinite descent: Suppose that √5 is rational, and express it in lowest possible terms (i.e., as a fully reduced fraction) as mn for natural numbers m and n. Then √5 can be expressed in lower terms as 5n − 2mm − 2n, which is a contradiction.