Problem:
Let , and be positive real numbers such that . Prove that at least one of , or is greater than .
Solution
Solution:
Wlog assume . Therefore . Now for a proof by contradiction, assume and . Since , it follows that and therefore . However this implies:
which is a contradiction.
As all three terms are positive, at least one must be less than or equal to and, without loss of generality, we can assume . But then and at least one of or must be greater than . Since we're done.
By the AM-GM inequality we have and thus , which rearranges to give . Now wlog and so
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