1992 IMO Problems/Problem 5
Problem
Let be a finite set of points in three-dimensional space. Let
,
,
, be the sets consisting of the orthogonal projections of the points of
onto the
-plane,
-plane,
-plane, respectively. Prove that
where denotes the number of elements in the finite set
. (Note: The orthogonal projection of a point onto a plane is the foot of the perpendicular from that point to the plane)
Solution
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