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In the three dimensional figure below, is there a way to prove that $$ \angle MNK = 90^ \circ $$


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up vote 3 down vote accepted

If I understand the labeling correctly, I don't think so—I don't see anything that prevents $\triangle MNL$ from "falling over" to land flat in the plane of $\triangle KNL$ (or being at any angle in between).

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