Hi Kevin,

The basic idea of a "proof" is just that you need to explain why something is true. Since we often wants students to explain their work for every problem, I often like describing a proof as a question where you already know the answer!

As an example (one part of 3.21) you need to prove that angles 2 and 8 (alternate exterior angles) are equal in the diagram below:

Try to explain why this is true (prove) this using only corresponding angles and vertical angles. Hint: This is not too hard!