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**1. Homework Statement**

So we're suppose to show that the intervals connecting vertices of a tetrahedron with centers of gravity of opposite sides interesect at one point, namely the center of gravity of a normal tetrahedron. The center of gravity is (P+Q+R+S)/4.

**2. Homework Equations**

All I know is we have to somehow work with vectors so we at the end, we end up with result.

I really have no idea how to start. Please help :(