How to: determine if the side lengths could form a triangle. use an inequality to prove your answer
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We need to simply use the triangle inequality Theorem, This theorem state that the sum of the two side lengths of a triangle must always be greater than the third side.
Now let's check from the given lengths
16 + 21 = 37 and 37 is less than 39 which is the third side
Hence, it cannot form a triangle