Maths Olympiad Prep

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Algebra Difficulty 5.0 AIME, harder Prove it China

The solution set of the inequality x32x24x+3<0|x|^3 - 2x^2 - 4|x| + 3 < 0 is
________.

Solution

Notice that x=3|x| = 3 is a root of the equation x32x24x+3=0|x|^3 - 2x^2 - 4|x| + 3 = 0. Then the original inequality can be rewritten as (x3)(x2+x1)<0(|x| - 3)(|x|^2 + |x| - 1) < 0, that is
(x3)(x1+52)(x152)<0. (|x| - 3)\left(|x| - \frac{-1 + \sqrt{5}}{2}\right)\left(|x| - \frac{-1 - \sqrt{5}}{2}\right) < 0.
Since x152>0|x| - \frac{-1 - \sqrt{5}}{2} > 0, then 1+52<x<3\frac{-1 + \sqrt{5}}{2} < |x| < 3.
So the solution set is (3,512)(512,3)(-3, -\frac{\sqrt{5} - 1}{2}) \cup (\frac{\sqrt{5} - 1}{2}, 3).

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