Proof in the attachment below.
Because L is the midpoint of KM, we know that KL = LM. Because M is the midpoint of LN, we know that LM = LN. Therefore, KL = LM = LN QED
To prove that KL = MN, we use the definitions of midpoints. Since L is the midpoint of KM, and M is the midpoint of LN, the resulting lengths show that KL equals MN. The relationships among the line segments confirm this conclusion.
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