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It might help to write out the lengths of the sides of each triangle in a set.
In the first set, try comparing the shortest side of the first triangle with the shortest side of the third triangle. What do you notice? How about comparing the two "middle length" sides?
Can you use this to work out what the longest side length is in the third triangle?
Number the Sides
Age 7 to 11
Challenge Level 
It might help to write out the lengths of the sides of each triangle in a set.
In the first set, try comparing the shortest side of the first triangle with the shortest side of the third triangle. What do you notice? How about comparing the two "middle length" sides?
Can you use this to work out what the longest side length is in the third triangle?
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