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Algebra Difficulty 5.0 AIME Prove it JBMO

Problem:
Let aa, bb, cc be positive real numbers such that a2+b2+c2=48a^{2} + b^{2} + c^{2} = 48. Prove
a22b3+16+b22c3+16+c22a3+16242 a^{2} \sqrt{2 b^{3} + 16} + b^{2} \sqrt{2 c^{3} + 16} + c^{2} \sqrt{2 a^{3} + 16} \leq 24^{2}
When does equality hold?

Solution

Solution:
Observe that 2x2+16=2(x2+8)=2(x+2)(x3=2x+4)2x^{2} + 16 = 2(x^{2} + 8) = 2(x + 2)(x^{3} = 2x + 4). From AM-GM:
2x3+16=(2x+4)(x32x+4)2x+4+x32x+42=x2+82 \sqrt{2x^{3} + 16} = \sqrt{(2x + 4)(x^{3} - 2x + 4)} \leq \frac{2x + 4 + x^{3} - 2x + 4}{2} = \frac{x^{2} + 8}{2}
By adding the inequality (1) obtained for x=ax = a, x=bx = b and x=cx = c it suffices to prove:
a3b3+8a3+b3c3+8b3+c3c3+8c32243, a^{3} b^{3} + 8a^{3} + b^{3} c^{3} + 8b^{3} + c^{3} c^{3} + 8c^{3} \leq 2 \cdot 24^{3},
Since a3b3+b3c3+c3a3(a3+b3+c3)33a^{3} b^{3} + b^{3} c^{3} + c^{3} a^{3} \leq \frac{\left(a^{3} + b^{3} + c^{3}\right)^{3}}{3}, using a3+b2+c2=48a^{3} + b^{2} + c^{2} = 48, we get the stated inequality.

Equality holds only when a=b=c=4a = b = c = 4.

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