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.

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.

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?

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