Question 195490
Which of the following could not be the lengths of the sides of a triangle? 
A: 5km, 2km, 4km
B: 3cm, 4cm, 5cm <== might be a triplet...
C:4in, 6in, 8in
D:6ft, 3ft, 9ft 
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This is not a Pythagorean problem.
You are not checking to see if you have a right triangle.
You want to know if the three sides could be the sides of a triangle (any
triangle).
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Criteria: The sum of every two sides must be greater than the 3rd side.
Example: sides of 1,2, and 7 could not be a triangle.
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Your Problem.
6,3,9 would not form a triangle.
Try it and you'll see.
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Cheers,
Stan H.