Generalization of triangle inequality in tetrahedron

Question

Let $(a,b)$, $(c,d)$, $(e,f)$ be oppisite edges of the tetrahedron. Our teacher said there's a generalization of triangle inequality: $$ab+cd>ef$$ but I don't know how prove it, and I'm not sure whether it's true because I didn't see any similar results on the Internet. Could someone give me some advise? Thank you!