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Imagine a tetrahedron made up of four triangles which have the same perimeter.
What might it look like? 

You could start to explore by choosing a triangle with a perimeter of 20, say, and use it as the base of your tetrahedron.

Can you find possibilities for the other three triangles if they all have perimeter 20 too?

Of course, if the triangles are all congruent, the perimeters will definitely be equal!
Can you find an example where the triangles aren't all congruent? Can you prove it?

Click below to see a diagram that might help:

Can you write down expressions for the different perimeters?
What can you deduce if you know these perimeters are all equal?

Can you make a tetrahedron out of any set of four congruent triangles?