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Algebra Difficulty 5.6 AIME, harder Prove it Taiwan

Given that a,b,c,da, b, c, d are non-negative real numbers, find the solutions (a,b,c,d)(a, b, c, d) satisfying the following system of equations:
a2(b+c)(b+c+d)=b+cb+c+d3 a^2(b+c)(b+c+d) = \sqrt{b+c}\sqrt[3]{b+c+d}
b2(c+d)(c+d+a)=c+dc+d+a3 b^2(c+d)(c+d+a) = \sqrt{c+d}\sqrt[3]{c+d+a}
c2(d+a)(d+a+b)=d+ad+a+b3 c^2(d+a)(d+a+b) = \sqrt{d+a}\sqrt[3]{d+a+b}
d2(a+b)(a+b+c)=a+ba+b+c3 d^2(a+b)(a+b+c) = \sqrt{a+b}\sqrt[3]{a+b+c}

Solution

If one of a,b,c,da, b, c, d is 00, it is easy to deduce that a=b=c=d=0a = b = c = d = 0 is a solution, so in what follows we may assume a,b,c,d>0a, b, c, d > 0.
After rearranging the original equations, we obtain
a12(b+c)3(b+c+d)4=1 a^{12}(b+c)^3(b+c+d)^4 = 1
b12(c+d)3(c+d+a)4=1 b^{12}(c+d)^3(c+d+a)^4 = 1
c12(d+a)3(d+a+b)4=1 c^{12}(d+a)^3(d+a+b)^4 = 1
d12(a+b)3(a+b+c)4=1 d^{12}(a+b)^3(a+b+c)^4 = 1
Consider xx to be the largest among the 1616 numbers {xyx,ya,b,c,d}\left\{\frac{x}{y}\mid x, y \in a, b, c, d\right\}, and without loss of generality assume ab=x1\frac{a}{b} = x \ge 1. Then
c+db+cx \frac{c+d}{b+c} \le x
c+d+ab+c+dx \frac{c+d+a}{b+c+d} \le x
Then
1=a12(b+c)3(b+c+d)4b12(c+d)3(c+d+a)4x121x31x4=x7 1 = \frac{a^{12}(b+c)^3(b+c+d)^4}{b^{12}(c+d)^3(c+d+a)^4} \ge x^{12} \frac{1}{x^3} \frac{1}{x^4} = x^7
Therefore x=1x = 1, that is a=b=c=da = b = c = d.

(Note: In the conditions given by the problem, a,b,c,da, b, c, d do not have full symmetry, but only a cyclic relationship, so the assumption that is truly without loss of generality should consider the three cases x=ab,ac,adx = \frac{a}{b}, \frac{a}{c}, \frac{a}{d}, but the above derivation still applies to all of them.)

Substituting back into the original equations, we solve to get (a,b,c,d)=(0,0,0,0)(a, b, c, d) = (0, 0, 0, 0) or (119/648,119/648,119/648,119/648)\left(\frac{1}{\sqrt{19/648}}, \frac{1}{\sqrt{19/648}}, \frac{1}{\sqrt{19/648}}, \frac{1}{\sqrt{19/648}}\right)

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from the original; metadata (topic, difficulty) added by this project.