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Hkg TST

Geometry Difficulty 7.5 National Olympiad, round 2 Prove it Hong Kong

Is it possible to choose 2424 distinct points in the space such that no three of them lie on the same line, and choose 20192019 distinct planes in a way that each plane passes through at least 33 of the chosen points and each triple of points belongs to one of the chosen planes?

Solution

Solution:

No. Suppose such a choice exists. Let n1,n2,…,n2019n_1, n_2, \dots, n_{2019} be the number of points on the planes respectively. Note that nj≥3n_j \ge 3 for 1≤j≤20191 \le j \le 2019. It is clear that
2024=(243)=(n13)+(n23)+⋯+(n20193). 2024 = \binom{24}{3} = \binom{n_1}{3} + \binom{n_2}{3} + \dots + \binom{n_{2019}}{3}.

We first show nj≤4n_j \le 4 for all 1≤j≤20191 \le j \le 2019. Indeed, if at least one is greater than 44, then
2024=(n13)+(n23)+⋯+(n20193)≥1+1+⋯+1⏟2018 times+(53)=2028, 2024 = \binom{n_1}{3} + \binom{n_2}{3} + \dots + \binom{n_{2019}}{3} \ge \underbrace{1+1+\dots+1}_{2018 \text{ times}} + \binom{5}{3} = 2028,
which is impossible.
Now suppose among all the njn_j's, aa of them are equal to 33 and bb of them are equal to 44. Then we have a+b=2019a+b = 2019 and
a+4b=a(33)+b(43)=2024. a + 4b = a\binom{3}{3} + b\binom{4}{3} = 2024.
This gives b=(a+4b)−(a+b)3=53b = \frac{(a+4b) - (a+b)}{3} = \frac{5}{3}, which is again impossible. Therefore, such a choice does not exist.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.