Arrange students in rows by columns. It is known that for any two columns, the number of occasions that two students in the same row are of the same sex does not exceed . Prove that the number of boy students does not exceed .
Solution
Let be the number of boy students in the \text{th}75 - a_i\sum_{i=1}^{22} \left( C_{a_i}^2 + C_{75-a_i}^2 \right) \le 11 \times C_{75}^2\sum_{i=1}^{22} (a_i^2 - 75a_i) \le -30,525\sum_{i=1}^{22} (2a_i - 75)^2 \le 1,650$. Using Cauchy's Inequality, we have
Then , and . That means the number of boy students does not exceed .
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