Why do this
problem?

It is instructive as an example of proof using the contrapositive of the statement that must be proved.

Possible approach

The class might discuss the proof in pairs before having a class discussion of the proof. It is very short but needs to be carefully argued.

Key question

What if the statement we are trying to prove were not true?

Possible support

Proof Sorter

It is instructive as an example of proof using the contrapositive of the statement that must be proved.

Possible approach

The class might discuss the proof in pairs before having a class discussion of the proof. It is very short but needs to be carefully argued.

Key question

What if the statement we are trying to prove were not true?

Possible support

Proof Sorter