Maths Olympiad Prep

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Algebra Difficulty 6.0 AIME, harder Prove it

60. Given a,bR+a, b \in \mathbf{R}^{+}, satisfying a2+ab+b2>3a^{2}+a b+b^{2}>3, prove: a2+aba^{2}+a b and b2+abb^{2}+a b at least one is greater than 2.

Solution

60. Proof: Without loss of generality, let aba \geqslant b, then a2+abb2+aba^{2}+a b \geqslant b^{2}+a b. Now we prove a2+ab>2a^{2}+a b>2. If a2+ab2a^{2}+a b \leqslant 2, then

and
b2a2ab \leqslant \frac{2-a^{2}}{a}

Therefore
31,b>131, b>1

Thus
11,thiscontradictstheassumption, this contradicts the assumption a>b.Hence. Hence a^{2}+a b>2$.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.