Maths Olympiad Prep

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Algebra Difficulty 4.9 AIME Prove it Philippines

Problem:
Find the largest real number xx such that x3+4x3=1\sqrt[3]{x} + \sqrt[3]{4-x} = 1.

Solution

Solution:
Cubing both sides of the given equation yields
x+3x(4x)3(x3+4x3)+4x=1, x + 3 \sqrt[3]{x(4-x)} (\sqrt[3]{x} + \sqrt[3]{4-x}) + 4 - x = 1,
which then becomes
4+3x(4x)3=1 4 + 3 \sqrt[3]{x(4-x)} = 1
or
x(4x)3=1. \sqrt[3]{x(4-x)} = -1.
Cubing both sides of this equation gives
x(4x)=1 x(4-x) = -1
or
x24x=1. x^2 - 4x = 1.
This means
(x2)2=5 (x-2)^2 = 5
so
x=2±5 x = 2 \pm \sqrt{5}
and the largest real solution is x=2+5x = 2 + \sqrt{5}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.