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Algebra Difficulty 6.6 National olympiad Find the answer

Find all triples (a,b,c)(a, b, c) of real numbers such that
a2+ab+c=0, a^2 + ab + c = 0,
b2+bc+a=0,b^2 + bc + a = 0,
c2+ca+b=0.c^2 + ca + b = 0.

A number or a short expression. Spacing and $ signs are ignored.

Solution

To find all triples (a,b,c)(a, b, c) of real numbers satisfying the system of equations:

1.a2+ab+c=0,2.b2+bc+a=0,3.c2+ca+b=0, \begin{align*} 1. \quad & a^2 + ab + c = 0, \\ 2. \quad & b^2 + bc + a = 0, \\ 3. \quad & c^2 + ca + b = 0, \end{align*}

we proceed by considering possible symmetric solutions first and also evaluate the possibility of all variables being zero.

### Step 1: Checking the trivial solution (0,0,0)(0, 0, 0)

Substituting a=0a = 0, b=0b = 0, c=0c = 0 into the equations, we find:

02+00+0=0,02+00+0=0,02+00+0=0. \begin{align*} 0^2 + 0 \cdot 0 + 0 &= 0, \\ 0^2 + 0 \cdot 0 + 0 &= 0, \\ 0^2 + 0 \cdot 0 + 0 &= 0. \end{align*}

Since all the equations are satisfied, (0,0,0)(0, 0, 0) is a valid solution.

### Step 2: Considering symmetric solutions where a=b=c=xa = b = c = x

Assume a=b=c=x a = b = c = x . Then the equations become:

x2+xx+x=02x2+x=0,x2+xx+x=02x2+x=0,x2+xx+x=02x2+x=0. \begin{align*} x^2 + x \cdot x + x &= 0 \quad \Rightarrow \quad 2x^2 + x = 0, \\ x^2 + x \cdot x + x &= 0 \quad \Rightarrow \quad 2x^2 + x = 0, \\ x^2 + x \cdot x + x &= 0 \quad \Rightarrow \quad 2x^2 + x = 0. \end{align*}

Each equation simplifies to:

2x2+x=0 2x^2 + x = 0

Factoring gives:

x(2x+1)=0 x(2x + 1) = 0

Thus, the solutions for xx are x=0x = 0 or x=12x = -\frac{1}{2}.

- For x=0x = 0, this corresponds to the solution (0,0,0)(0, 0, 0).
- For x=12x = -\frac{1}{2}, this gives the solution (12,12,12)\left( -\frac{1}{2}, -\frac{1}{2}, -\frac{1}{2} \right).

Both solutions satisfy the original equations, since substituting back verifies equality:

(12)2+(12)(12)+(12)=14+1412=0,(12)2+(12)(12)+(12)=0,(12)2+(12)(12)+(12)=0. \begin{align*} \left(-\frac{1}{2}\right)^2 + \left(-\frac{1}{2}\right)\left(-\frac{1}{2}\right) + \left(-\frac{1}{2}\right) &= \frac{1}{4} + \frac{1}{4} - \frac{1}{2} = 0, \\ \left(-\frac{1}{2}\right)^2 + \left(-\frac{1}{2}\right)\left(-\frac{1}{2}\right) + \left(-\frac{1}{2}\right) &= 0, \\ \left(-\frac{1}{2}\right)^2 + \left(-\frac{1}{2}\right)\left(-\frac{1}{2}\right) + \left(-\frac{1}{2}\right) &= 0. \end{align*}

### Final Solutions

Thus, the complete set of solutions for (a,b,c)(a, b, c) is:

(0,0,0)and(12,12,12) \boxed{(0, 0, 0) \quad \text{and} \quad \left(-\frac{1}{2}, -\frac{1}{2}, -\frac{1}{2}\right)}

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.