Given that a,b,c,d are non-negative real numbers, find the solutions (a,b,c,d) satisfying the following system of equations: a2(b+c)(b+c+d)=b+c3b+c+d b2(c+d)(c+d+a)=c+d3c+d+a c2(d+a)(d+a+b)=d+a3d+a+b d2(a+b)(a+b+c)=a+b3a+b+c
Solution
If one of a,b,c,d is 0, it is easy to deduce that a=b=c=d=0 is a solution, so in what follows we may assume a,b,c,d>0. After rearranging the original equations, we obtain a12(b+c)3(b+c+d)4=1 b12(c+d)3(c+d+a)4=1 c12(d+a)3(d+a+b)4=1 d12(a+b)3(a+b+c)4=1 Consider x to be the largest among the 16 numbers {yx∣x,y∈a,b,c,d}, and without loss of generality assume ba=x≥1. Then b+cc+d≤x b+c+dc+d+a≤x Then 1=b12(c+d)3(c+d+a)4a12(b+c)3(b+c+d)4≥x12x31x41=x7 Therefore x=1, that is a=b=c=d.
(Note: In the conditions given by the problem, a,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=ba,ca,da, 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) or (19/6481,19/6481,19/6481,19/6481)
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