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Difficulty: 4/102024 IOQM 2024 (Q11)

The positive real numbers satisfy:

What is the value of ?

Guide / Hint

Hint 1: Try adding the two equations together. Look at the resulting numerators and denominators.

Hint 2: After adding, the equation simplifies to . Apply the AM-GM inequality to this expression.

Hint 3: Use the AM-GM equality condition to show that . Find their common value using the second equation, then calculate .

Solution

Step 1: Let the two given equations be:

Step 2: Rearrange the terms of equation (2) to group them with the corresponding denominators in (1):

Step 3: Add equations (1) and (2) together:

Step 4: Since are positive real numbers, all terms in the sum are positive. By the Arithmetic Mean-Geometric Mean (AM-GM) inequality:

Step 5: Since the sum is exactly , the equality condition of the AM-GM inequality must hold. This requires all three terms to be equal to :

This implies:

for some positive constant .

Step 6: Substitute these into equation (2):

Step 7: Now solve for :

Step 8: Finally, calculate the requested sum:

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