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Difficulty: 4/102024 NMTC 2024 (QII-54)

For how many positive integers does there exist at least one positive integer such that mn < + n?

Guide / Hint

Hint 1: Rearrange the inequality to .

Hint 2: Recall that must be positive integers, so and .

Hint 3: Note that if we choose , the inequality simplifies to , which is true for all positive integers . This implies there are infinitely many such .

Solution

Step 1 (Analyze the Inequality): We are looking for positive integers such that there exists at least one positive integer satisfying:

Step 2 (Test a Specific Value for n): Let's test the smallest positive integer for , which is . Substituting into the inequality:

Step 3 (Evaluate the Resulting Inequality): The inequality is a fundamental arithmetic fact that is strictly true for all real numbers, and thus true for all positive integers .

Step 4 (Conclusion): Since choosing satisfies the inequality for every positive integer , there exists at least one positive integer (namely ) for every single positive integer . Therefore, there are infinitely many such positive integers .

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    2024 NMTC 2024 QII-54 - Olympiad Math Olympiad Question | Leminno