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# Triangle Midpoints

### Why do this problem ?

### Possible approach:

*This printable worksheet may be useful: Triangle Midpoints.*

To start with there is some thinking to do about the way in which knowledge of the mid points is sufficient to fix the triangle (surprising?). Show the question and ask for ideas about possible approaches, things the group notices and things that might be true but they/others would need to be convinced about.
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Age 14 to 16

Challenge Level

- Problem
- Getting Started
- Student Solutions
- Teachers' Resources

This problem has a number of possible routes to a solution. It could be used as the focus for applying or learning some standard 'ruler and compasses' techniques but it is also possible to answer the problem without them. One way to work at this is backwards, a good problem solving strategy! Try to leave plenty
of room for different methods to emerge.

To start with there is some thinking to do about the way in which knowledge of the mid points is sufficient to fix the triangle (surprising?). Show the question and ask for ideas about possible approaches, things the group notices and things that might be true but they/others would need to be convinced about.

One way forward might be to start from what the group knows or is relatively confident about. For example, being able to find the midpoints if you start with the triangle. It is this idea of working backwards, from what you know, that you are aiming to draw out of discussions.

Start by asking the group to imagine a triangle (in their mind's eye).

- What does it look like?
- Is it regular?

Encourage non-regular visualisations but this is not essential at this stage.

Now put a point at the midpoint of one side, then a second side and, finally, the third side.

Join the points up.

- What do you notice? The answer might just be they they make a triangle but press for any other ideas which might emerge that you could pursue.

You might wish to repeat this process. When ready, encourage groups to draw triangles of their own, to find midpoints and construct inner "midpoint triangles". After some time, share different results and anything the class has noticed, including a discussion of (and possibly practice at) how to find the midpoints. Learners may use paper folding or approximation so there is useful time to be
spent on finding a midpoint accurately and discussing the alternatives.

Return to the problem, giving plenty of time for learners to find their own way. If additional stimulus is needed share ideas so far. You might offer the attached document to think about. To stimulate thinking about how you might convince someone that all four triangles are congruent this diagram might also be helpful.

Use time to discuss what different groups notice and how it might help. Encourage rigour.

This problem creates a good opportunity to practise some common classroom geometry constructions, for example: a perpendicular bisector of a given line, angle bisector centre for the circumcircle of a given triangle, drawing a line through a given point parallel to a given line.

It is particularly important to promote a lively discussion of possible reasons for a construction method's validity.

- What does working backwards from what you know reveal about the problem that could be helpful?
- How do you know...? How can you convince us?
- Why does the construction work ?

Establish the relationships with paper folding and cutting and use this to see why triangles are congruent and therefore why particular approaches are valid.

What about midpoints of sides of other polyhedra?

The largest square which fits into a circle is ABCD and EFGH is a square with G and H on the line CD and E and F on the circumference of the circle. Show that AB = 5EF. Similarly the largest equilateral triangle which fits into a circle is LMN and PQR is an equilateral triangle with P and Q on the line LM and R on the circumference of the circle. Show that LM = 3PQ

Two ladders are propped up against facing walls. The end of the first ladder is 10 metres above the foot of the first wall. The end of the second ladder is 5 metres above the foot of the second wall. At what height do the ladders cross?