Next time you are teaching this course, you do not repeat your mistake! You ask yourself: for what prime numbers does this “even-odd” proof work? You easily see that it works for all integers congruent to 3 modulo 4. It is clear what you have to
do. The smallest prime number larger than 2 but not congruent to 3 modulo 4 is 5.
Thus, you ask your students to prove that √5 is irrational.