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'Number the Sides' printed from http://nrich.maths.org/
The triangles in this set are 'similar':
'Similar' means that the triangles are exactly the same shape, but
not the same size. The sides are in the same ratio to each other.
(Note that these triangles are not drawn to scale.)
What can you say about the length of the side of the third triangle
which is marked with a question mark? Of course the triangles could
be different ways up, too:
There are five more sets of similar triangles below. Can you work
out the lengths of the sides marked with a question mark?