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Difficulty: 5/102023 NMTC 2023 (QIII-1)

Let a, and be distinct nonzero real numbers such that + 1/b = + 1/c = + 1/a. Prove that |abc| = 1.

Guide / Hint

Hint 1: Rearrange the equations to relate differences to products: . This gives .

Hint 2: Write similar equations for and .

Hint 3: Multiply the three expressions for together. Notice how the difference terms cancel out, leaving .

Solution

Step 1 (Formulate pairwise equations): We are given distinct nonzero real numbers such that:

We can form three separate equations:

Step 2 (Multiply the three equations): Multiplying the left-hand sides and right-hand sides of these three equations together:

Since are distinct, their differences are non-zero, so we can cancel them:

Step 3 (Conclusion): The value is exactly 1.

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