Archive | Proofs

How to Use Definitions in Geometry

Definitions are TWICE as useful as standard theorems in geometric proofs. This post explains why, showing that definitions are, in a certain sense, very much like “reversible coats.”

How to See Why the Divisibility Trick for 3 Works

There’s a nifty trick for finding out if 3 divides evenly into a number. This little proof shows why the trick works.