a squared plus b squared equals c squared - A math rule that helps you find the longest side of a triang

A math rule that helps you find the longest side of a triangle if you're too lazy to do anything else and just want to prove your teacher wrong.

I used this rule to prove my teacher wrong. She got mad and I got a cookie.

I used this rule in a test and got an A. My teacher got sad.

I forgot this rule and got a D. My teacher got happy.