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Algebra Difficulty 7.1 National Olympiad, round 2 Prove it JBMO

Problem:
Find all triples (a,b,c)(a, b, c) of real numbers such that the following system holds:
{a+b+c=1a+1b+1ca2+b2+c2=1a2+1b2+1c2 \left\{\begin{array}{l} a+b+c=\frac{1}{a}+\frac{1}{b}+\frac{1}{c} \\ a^{2}+b^{2}+c^{2}=\frac{1}{a^{2}}+\frac{1}{b^{2}}+\frac{1}{c^{2}} \end{array}\right.

Solution

Solution:
First of all if (a,b,c)(a, b, c) is a solution of the system then also (a,b,c)(-a,-b,-c) is a solution. Hence we can suppose that abc>0a b c>0. From the first condition we have
a+b+c=ab+bc+caabc a+b+c=\frac{a b+b c+c a}{a b c}
Now, from the first condition and the second condition we get
(a+b+c)2(a2+b2+c2)=(1a+1b+1c)2(1a2+1b2+1c2) (a+b+c)^{2}-\left(a^{2}+b^{2}+c^{2}\right)=\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)^{2}-\left(\frac{1}{a^{2}}+\frac{1}{b^{2}}+\frac{1}{c^{2}}\right)
The last one simplifies to
ab+bc+ca=a+b+cabc a b+b c+c a=\frac{a+b+c}{a b c}
First we show that a+b+ca+b+c and ab+bc+caa b+b c+c a are different from 00. Suppose on contrary then from relation (1) or (2) we have a+b+c=ab+bc+ca=0a+b+c=a b+b c+c a=0. But then we would have
a2+b2+c2=(a+b+c)22(ab+bc+ca)=0 a^{2}+b^{2}+c^{2}=(a+b+c)^{2}-2(a b+b c+c a)=0
which means that a=b=c=0a=b=c=0. This is not possible since a,b,ca, b, c should be different from 00. Now multiplying (1) and (2) we have
(a+b+c)(ab+bc+ca)=(a+b+c)(ab+bc+ca)(abc)2 (a+b+c)(a b+b c+c a)=\frac{(a+b+c)(a b+b c+c a)}{(a b c)^{2}}
Since a+b+ca+b+c and ab+bc+caa b+b c+c a are different from 00, we get (abc)2=1(a b c)^{2}=1 and using the fact that abc>0a b c>0 we obtain that abc=1a b c=1. So relations (1) and (2) transform to
a+b+c=ab+bc+ca. a+b+c=a b+b c+c a.
Therefore,
(a1)(b1)(c1)=abcabbcca+a+b+c1=0. (a-1)(b-1)(c-1)=a b c-a b-b c-c a+a+b+c-1=0.
This means that at least one of the numbers a,b,ca, b, c is equal to 11. Suppose that c=1c=1 then relations (1) and (2) transform to a+b+1=ab+a+bab=1a+b+1=a b+a+b \Rightarrow a b=1. Taking a=ta=t then we have b=1tb=\frac{1}{t}. We can now verify that any triple (a,b,c)=(t,1t,1)(a, b, c)=\left(t, \frac{1}{t}, 1\right) satisfies both conditions. tR{0}t \in \mathbb{R} \setminus\{0\}. From the initial observation any triple (a,b,c)=(t,1t,1)(a, b, c)=\left(t, \frac{1}{t},-1\right) satisfies both conditions. tR{0}t \in \mathbb{R} \setminus\{0\}. So, all triples that satisfy both conditions are (a,b,c)=(t,1t,1),(t,1t,1)(a, b, c)=\left(t, \frac{1}{t}, 1\right),\left(t, \frac{1}{t},-1\right) and all permutations for any tR{0}t \in \mathbb{R} \setminus\{0\}.

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