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Algebra Difficulty 6.3 National olympiad Prove it Belarus

Find all triples (x;y;z)(x; y; z) of real numbers satisfying the system
{(x+1)(x2+1)=y3+1,(y+1)(y2+1)=z3+1,(z+1)(z2+1)=x3+1. \begin{cases} (x+1)(x^2+1) = y^3+1, \\ (y+1)(y^2+1) = z^3+1, \\ (z+1)(z^2+1) = x^3+1. \end{cases}

Solutions — 2

Solution 1

It is easy to see that the triples (0;0;0)(0; 0; 0) and (1;1;1)(-1; -1; -1) satisfy the system. We will show that there are no other solutions.
From the equations
(x+1)(x2+1)=y3+1,(1) (x+1)(x^2+1) = y^3+1, \quad (1)
(y+1)(y2+1)=z3+1,(2) (y+1)(y^2+1) = z^3+1, \quad (2)
(z+1)(z2+1)=x3+1(3) (z+1)(z^2+1) = x^3+1 \quad (3)
it follows that if x=1x = -1, then, from (1) y=1y = -1, and from (2) z=1z = -1. Similarly, if x=0x = 0, then y=0y = 0 and z=0z = 0. Further assume that none of xx, yy and zz equals 1-1 or 00.
Factor the right-hand sides of (1), (2), (3) and multiply the results:
(x+1)(x2+1)(y+1)(y2+1)(z+1)(z2+1)=(y+1)(y2y+1)(z+1)(z2z+1)(x+1)(x2x+1). (x+1)(x^2+1)(y+1)(y^2+1)(z+1)(z^2+1) = (y+1)(y^2-y+1)(z+1)(z^2-z+1)(x+1)(x^2-x+1).
Reducing by (x+1)(y+1)(z+1)0(x+1)(y+1)(z+1) \ne 0, we get
(x2+1)(y2+1)(z2+1)=(x2x+1)(y2y+1)(z2z+1).() (x^2+1)(y^2+1)(z^2+1) = (x^2-x+1)(y^2-y+1)(z^2-z+1). \quad (*)
Note that if x<1x < -1, then, according to (1), y<1y < -1 and, according to (2), z<1z < -1. In this case all factors in the right-hand side of (*) are greater than corresponding factors in the left-hand side — a contradiction.
Now factor the left-hand sides of equations (1), (2) and (3), and subtracting 1 from both sides of these equations, we obtain the equations
x(x2+x+1)=y3,(4) x(x^2 + x + 1) = y^3, \quad (4)
y(y2+y+1)=z3,(5) y(y^2 + y + 1) = z^3, \quad (5)
z(z2+z+1)=x3.(6) z(z^2 + z + 1) = x^3. \quad (6)
Note that if x>0x > 0, then, according to (4), y>0y > 0 and, according to (5), z>0z > 0. In this case all factors in the right-hand side of (*) are less than corresponding factors in the left-hand side — a contradiction.
It remains to consider the case, when x,yx, y and zz belong to the interval (1;0)(-1; 0) and, in particular, are negative. Again, all factors in the right-hand side of (*) are greater than corresponding factors in the left-hand side. So, there are no other solutions.

Solution 2

Consider the values of polynomials x3+x2+xx^3 + x^2 + x and x3x^3 on the intervals (0;+)(0; +\infty), (;1)(-\infty; -1) and (1;0)(-1; 0).
If x>0x > 0, then x3+x2+x>x3>0x^3 + x^2 + x > x^3 > 0. From the equations of the system successively obtain the inequalities y>x>0y > x > 0, z>y>0z > y > 0 and x>z>0x > z > 0, whence x>zx > z.
If x<1x < -1, then x3+x2<0x^3 + x^2 < 0 and x2+x>0x^2 + x > 0, hence 1>x3+x2+x>x3-1 > x^3 + x^2 + x > x^3. From the equations of the system we obtain the inequalities 1>y>x-1 > y > x, 1>z>y-1 > z > y and 1>x>z-1 > x > z, and again x>zx > z.
If x(1;0)x \in (-1; 0), then x3+x2>0x^3 + x^2 > 0 and x2+x<0x^2 + x < 0, therefore 1<x3+x2+x<x3<0-1 < x^3 + x^2 + x < x^3 < 0. From the system we obtain the inequalities 1>y>x-1 > y > x, 1>z>y-1 > z > y and 1>x>z-1 > x > z, whence again x>zx > z.
In all three cases we got a contradiction and the two remaining variants x=0x = 0 and x=1x = 1 easily lead us to the triples from the answer.

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