Difference between revisions of "1992 IMO Problems/Problem 5"
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Adding all <math>|Z_{i}|</math> we get: | Adding all <math>|Z_{i}|</math> we get: | ||
− | <math>\sum_{i=1}^{n}|Z_{i}| \le \sqrt{|S_{z}|}\sum_{i=1}^{n}\sqrt{a_{i}b_{i}}</math> | + | <math>\sum_{i=1}^{n}|Z_{i}| \le \sqrt{|S_{z}|}\sum_{i=1}^{n}\sqrt{a_{i}b_{i}}\;</math>[Equation 6] |
{{alternate solutions}} | {{alternate solutions}} |
Revision as of 13:34, 12 November 2023
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
Let be planes with index such that that are parallel to the -plane that contain multiple points of on those planes such that all points of are distributed throughout all planes according to their -coordinates in common.
Let be the number of unique projected points from each to the -plane
Let be the number of unique projected points from each to the -plane
This provides the following:
[Equation 1]
We also know that
[Equation 2]
Since be the number of unique projected points from each to the -plane,
if we add them together it will give us the total points projected onto the -plane.
Therefore,
[Equation 3]
likewise,
[Equation 4]
We also know that the total number of elements of each is less or equal to the total number of elements in
That is,
[Equation 5]
Multiplying [Equation 1] by [Equation 5] we get:
Therefore,
Adding all we get:
[Equation 6]
Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.