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'Inside Triangles' printed from http://nrich.maths.org/
Why do this problem?
is accessible to all pupils but has the scope to be extended in many directions. In order to make a start, children will need to be familiar with properties of triangles, but drawing triangles on the grid will help them to clarify for themselves what they understand by the term "triangle". In order to find all the
possible triangles, pupils will need to work in a systematic way.
You may want to begin this task with the whole class and, this way, the notion of "different" will come up quite quickly. How is the group going to define "different"? This is a great discussion point and one where there isn't a right or a wrong answer. You could decide to count triangles which could be picked up and placed exactly on top of another triangle as the same. Or, you could decide
that they are different if they are in a different orientation on the grid. The former suggestion makes a more manageable number to count!
It might help to suggest working in pairs on this activity so that children are checking they haven't duplicated triangles. Learners might find it helpful to use a pegboard and/or to draw their triangles on this sheet
The interactivity, if projected or used on an IWB, allows findings to be shared easily. In addition to checking that the triangles are indeed all different, a plenary could focus on how the children know that they have found them all, which is quite a challenge. Listen out for learners who have a 'system' of some description which they follow to make sure they don't miss any
out. Alternatively, you could ask children to draw each triangle on a different grid and try to group the triangles that have been found. That way, the imposed method of grouping will help to identify any that have been omitted.
Tell me about the way you're working.
How will you remember which triangles you've found?
How do you know that your triangles are all different from each other?
How do you know that you have found them all?
In order to extend the problem, pupils could be asked to find triangles with three spots inside them or no spots inside ... Differently sized grids could be drawn and compared. You could sort the triangles across differently sized grids, for example all right-angled triangles together, or all triangles which are the same shape but different sizes together.
Having a range of different equipment available for children to use to tackle this problem (e.g. pegboards, grids
on paper, the interactivity) will help everyone get started.