The concurrency of the heights of a tetrahedron

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The sentence says that the opposite edges of the tetrahedron ABCD are perpendicular and the fact that $AB^2+CD^2=AC^2+BD^2=BC^2+AD^2$ and I need to prove that the heights of the tetrahedron are concurrent, but I am stuck. I need a vectorial proof.The picture of my solution so far