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Why do this problem?
allows children to make important discoveries about triangles for themselves through representing the sides of triangles with strips from a construction kit. They will have the opportunity to find out which combinations of lengths cannot be made into triangles and why this is so. They should also
discover how rigid triangles are and that three fixed lengths can only be arranged in one way (unlike other polygons such as quadrilaterals where the angles can be changed).
This investigation requires plenty of equipment and space, so is probably best done in a small group.
You could start by demonstrating with the interactivity in this problem
to show the children what can be done with three rods or sticks.
After they have been introduced to the problem, encourage the children to work on making different triangles with three lengths of strips. If they are mature enough to work in pairs, the children will gain from talking through their ideas with a partner. If they are working on a suitable plastic table the length of the rods (as numbers) can be written on the table with whiteboard markers to check
if any of the triangles made are repeats. Go on to using all four lengths when you judge the children are ready.
If you do not have access to a construction set or suitable rods, this sheet
will give you four copies of each strip in the colours in the problem. If they are printed out on card and laminated they can be laid flat on a table to make the triangles. This sheet
has uncoloured strips that can be photocopied. Paper copies can be pasted onto a backing sheet but make sure that the children have made a triangle before they have access to any glue or paste as they will probably be frustrated on finding that the third side does not fit!
In a plenary, children can show the triangles they have made and any three lengths that will not make a triangle. Invite them to explain why the three lengths cannot make a triangle, and look out for those learners who begin to make generalisations.
Have you made any triangles using this length of strip?
Have you made any triangles with all the sides the same colour/length?
Have you made any triangles with all the sides different colours/lengths?
Are you sure you haven't made one like that already?
Why do you think you can't make a triangle with those three sticks/rods?
Children who work quickly could be encouraged to find a way to show that they have discovered all of the possible triangles using four different lengths. They can be encouraged to compare the different triangles they have made and describe them to one another looking carefully at their characteristics.
Some learners might find it helpful to use just two lengths to start with. (Make sure that the longest one is shorter than two of the others put together lengthways.) You can make four different triangles with these. You can then add a third length.