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Algebra Difficulty 5.9 AIME, harder Prove it Romania

Determine all triples of real numbers (a,b,c)(a, b, c) that satisfy simultaneously the equations:
a(b2+c)=c(c+ab),b(c2+a)=a(a+bc),c(a2+b)=b(b+ca). \begin{align*} a(b^2 + c) &= c(c + ab), \\ b(c^2 + a) &= a(a + bc), \\ c(a^2 + b) &= b(b + ca). \end{align*}

Solution

Let (a,b,c)(a, b, c) be a solution of the system. If one of the numbers a,b,ca, b, c is 00, e.g. if c=0c = 0, then c(a2+b)=a(b+ca)c(a^2 + b) = a(b + ca) leads to a=0a = 0, and similarly one gets b=0b = 0. Thus abc=0abc = 0 leads to a=b=c=0a = b = c = 0 which is indeed a solution. We now look for solutions with abc0abc \neq 0. We rewrite the equations
ab(bc)=c(ca)bc(ca)=a(ab)ca(ab)=b(bc). \begin{align*} ab(b - c) &= c(c - a) \\ bc(c - a) &= a(a - b) \\ ca(a - b) &= b(b - c). \end{align*}
It follows that a2b2c2(ab)(bc)(ca)=abc(ab)(bc)(ca)a^2b^2c^2(a - b)(b - c)(c - a) = abc(a - b)(b - c)(c - a).

Case 1: Among the numbers a,b,ca, b, c there exist (at least) two equal ones; say a=ba = b. We have
a2c+bc=b2+abcbc=b2c=b, a^2c + bc = b^2 + abc \Leftrightarrow bc = b^2 \Leftrightarrow c = b,
therefore in this case we obtain a=b=ca = b = c. Conversely, any such triple is a solution to the system.

Case 2: If abcaa \neq b \neq c \neq a, then abc=1abc = 1. We obtain ab(bc)=c(ca)(bc)=c2(ca)ab(b - c) = c(c - a) \Leftrightarrow (b - c) = c^2(c - a) and two more equations, similar to this one. It follows that
a3+b3+c3=ac2+ba2+cb2. a^3 + b^3 + c^3 = ac^2 + ba^2 + cb^2.
If a,b,c>0a, b, c > 0, from AM-GM it follows that a3+a3+b33ba2a^3 + a^3 + b^3 \ge 3ba^2 (with equality if and only if a=ba = b) which, added with two similar inequalities, leads to a3+b3+c3ac2+ba2+cb2a^3 + b^3 + c^3 \ge ac^2 + ba^2 + cb^2. We thus have equality in the previous inequality, hence a=b=ca = b = c.

If one of the variables is positive and the other two are negative, say a>0a > 0, b,c<0b, c < 0, then
b(c2+a)=a(a+bc)=a2+1>0a<0, b(c^2 + a) = a(a + bc) = a^2 + 1 > 0 \Rightarrow a < 0,
contradiction.

In conclusion, the only solutions are (a,b,c)=(x,x,x)(a, b, c) = (x, x, x), xR\forall x \in \mathbb{R}.

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