Napkin

Why do this problem

This problem offers a simple context which can generate lots of questions. Inviting learners to make conjectures and form convincing arguments. Demonstrating the similarity of triangles is relatively straight forward and calculating lengths offers opportunities for links with Pythagoras' theorem and ratios, bringing together important geometrical concepts.

Possible approach

Time to engage in, and become familiar with, the context is important. Early on, encourage learners to list and share what they notice, using large squares of paper on a display board can encourage discussion of key features and ideas and conjectures which they might explore.

Identify questions about the triangles that learners will work on.

As learners work this document may help them discuss possibilities and focus on some possible approaches.

It is likely that learners will arrive at results in different ways. These journeys and findings form opportunities to share and discuss good and elegant solutions and different ways of "seeing".
 

Key questions

• What do you think might be true?
• What do you know?
• What do you need to know?
• What mathematical ideas and techniques might be of use in order to answer that question?
   

Possible support
 

Possible extension

If the square paper napkin is folded so that the corner P does not coincide with the midpoint of an opposite edge, where would you place the fold for a 5, 12, 13 or an 8, 15, 17  or a 7, 24, 25 triangle?

Are any of these findings extendable to other quadrilaterals?