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Algebra Difficulty 5.2 AIME, harder Prove it Ukraine

Compare two numbers:
2008+2009+2009+2008 and 2008+2008+2009+2009 \sqrt{2008 + \sqrt{2009}} + \sqrt{2009 + \sqrt{2008}} \text{ and } \sqrt{2008 + \sqrt{2008}} + \sqrt{2009 + \sqrt{2009}}

Solution

Reformulate this task in the general case: for the positive distinct real numbers a,ba, b compare two numbers: A=a+b+b+aA = \sqrt{a+\sqrt{b}} + \sqrt{b+\sqrt{a}} and B=a+a+b+bB = \sqrt{a+\sqrt{a}} + \sqrt{b+\sqrt{b}}. Prove that A>BA > B.

Is equivalent to A2>B2a+bb+a>a+ab+bA^2 > B^2 \Leftrightarrow \sqrt{a+\sqrt{b}} \cdot \sqrt{b+\sqrt{a}} > \sqrt{a+\sqrt{a}} \cdot \sqrt{b+\sqrt{b}}.

After some simple calculations we have: aa+bb>ab+ba(ab)(ab)>0a\sqrt{a} + b\sqrt{b} > a\sqrt{b} + b\sqrt{a} \Leftrightarrow (a-b)(\sqrt{a}-\sqrt{b}) > 0 if aba \ne b.

This completes the proof.

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