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Algebra Difficulty 5.4 AIME, harder Prove it Silk Road Mathematics Competition

Prove the inequality
4(ab3+bc3+ca3)3(2+a+b+c+1a+1b+1c)2/3 4 \left( \sqrt[3]{\frac{a}{b}} + \sqrt[3]{\frac{b}{c}} + \sqrt[3]{\frac{c}{a}} \right) \le 3 \left( 2 + a + b + c + \frac{1}{a} + \frac{1}{b} + \frac{1}{c} \right)^{2/3}
for positive real numbers aa, bb and cc with abc=1abc = 1.

Solutions — 2

Solution 1

Let xx, yy, zz be positive real numbers such that a=x/ya = x/y, b=y/zb = y/z, c=z/xc = z/x. After substitution and simplifying the given inequality is transformed to the following one:
4(x+y+z)3xyz3(x+y)2/3(y+z)2/3(z+x)2/3 4(x + y + z)^{\sqrt{3}xyz} \le 3(x + y)^{2/3}(y + z)^{2/3}(z + x)^{2/3}
For triangle with the sides lengths u=x+yu = x + y, v=y+zv = y + z, w=z+xw = z + x the last inequality gives
4p(pu)(pv)(pw)33(uvw)2/3 4p\sqrt[3]{(p-u)(p-v)(p-w)} \le 3(uvw)^{2/3}
where pp is a semiperimeter of the triangle. By Heron's formula we obtain that the inequality is equivalent to:
(2p)2/3(33uvw4s)2/3=(33R)2/3 (2p)^{2/3} \le (3\sqrt{3}\frac{uvw}{4s})^{2/3} = (3\sqrt{3}R)^{2/3}
where SS is area and RR is a radius of the circumcircle of the triangle. But the inequality
u+v+w=2p33R u + v + w = 2p \le 3\sqrt{3}R
is well-known; there are several ways to prove that.

Solution 2

By raising to the third power the given inequality and taking into account the condition abc=1abc = 1 we eliminate radicals. After simplifying we get the following inequality:
114+30cyc(a+1a)+10cycab54cycba+27cyc(a2+1a2) 114 + 30 \sum_{cyc} \left(a + \frac{1}{a}\right) + 10 \sum_{cyc} \frac{a}{b} \le 54 \sum_{cyc} \frac{b}{a} + 27 \sum_{cyc} \left(a^2 + \frac{1}{a^2}\right)
The last inequality can be obtained from the following elementary inequalities:
2cycabcyc(a2+1a2),3cycba, 2 \sum_{cyc} \frac{a}{b} \le \sum_{cyc} \left(a^2 + \frac{1}{a^2}\right), \quad 3 \le \sum_{cyc} \frac{b}{a},
a+1a14(a2+1a2+6),2a2+1a2. a + \frac{1}{a} \le \frac{1}{4} \left(a^2 + \frac{1}{a^2} + 6\right), \quad 2 \le a^2 + \frac{1}{a^2}.

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