# Proofs: Problem 2

### Self-check

Are there any fractions in your proof? If so, go back and fiddle with
the equations. See the
hints.

Did you declare your variables at the start?

Did you start at the assumptions (not one step later) and end at the
conclusion (not one step before)?

Did you say explicitly when you were using the fact that s is not zero?

Now continue to the
model solution.