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Junior Macedonian Mathematical Olympiad

North Macedonia algebra

Problem

Let , and be positive real numbers such that . Prove that the following inequality holds When does equality hold?
Solution
Since it follows that , i.e. . We get that In the same way we prove that and

By adding (1), (2) and (3) we get the required inequality. Let us note that equality holds if and only if , i.e. . In the same way we get and .
Final answer
Equality holds if and only if a = b = c = 1.

Techniques

QM-AM-GM-HM / Power Mean