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Difficulty: 5/102020 IOQM 2020 (Q18)

If where are positive integers, , and , then what is the value of ?

Guide / Hint

Hint 1: Simplify the expression inside the square root by finding a common denominator: .

Hint 2: Show that the numerator is a perfect square , allowing the square root to simplify to .

Hint 3: Apply the partial fraction decomposition to telescope the sum from to , then compute .

Solution

Step 1: Simplify the term inside the square root:

Step 2: Notice that the numerator is a perfect square:

Step 3: Thus, the square root simplifies to:

Step 4: Use partial fractions to rewrite the fractional part:

Step 5: Sum this expression from to :

Step 6: The summation telescopes:

Step 7: Combine the terms:

Step 8: Comparing to , we get , , . These satisfy and .

Step 9: Finally, find :

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