### Why do this problem?

This
problem begins with a simple paper fold, which leads to an
elegant result. Learners need to estimate lengths and work
accurately to construct and measure lines. Once they have made a
conjecture it can be justified with geometrical arguments using
angles and ratio. To prove the results knowledge of similar
triangles is needed, but the problem could equally well be used as
an introduction to the idea of similarity.

### Possible approach

Start by giving everyone a square of paper which they can fold
according to the diagrams. Encourage them to measure as accurately
as possible the two sections of the diagonal which are formed.
Collect together some measurements and ask what the learners
notice. What do they think would happen with squares of different
sizes? In order to collect a lot of data in a short space of time,
small groups could create different sized squares and construct the
lines and measure them, sharing the results within their
group.

Once a pattern has emerged bring the class together and ask
what they have found. Some learners will be convinced that the
relationship will always hold because they have tried it with lots
of examples, so there is a good opportunity to discuss the
distinction between demonstration and mathematical proof.

In order to justify formally what they have noticed, learners
might find it useful to discuss what they know to be true, and what
a convincing argument shows to be true. This might be for example
that certain angles are equal.

Once pairs of equal angles are identified a route to a proof
becomes clearer.

The last part of the question asks about other quadrilaterals. Some
learners will be secure enough in the proof for squares to attempt
to prove or disprove the cases for other quadrilaterals straight
away without the need for accurate diagrams or folding.

### Key questions

What does it look like the folds are doing to the lines?

How could you verify what you think is true?

What other mathematics do you know that might be useful
here?

Does this work for other quadrilaterals?

### Possible extension

The problem

Take
a Square II builds on these ideas and extends them to different
fractions of the line.

### Possible support

The problem

Two
Ladders could be used to investigate similar triangles.