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USA Team Selection Test

Algebra Difficulty 7.3 National Olympiad, round 2 Find the answer

Find all triples (x,y,z) (x,y,z) of real numbers that satisfy the system of equations
x 3 3x 12y 50, y 3 12y 3z 2, z 3 27z 27x.\text{x 3 3x 12y 50, y 3 12y 3z 2, z 3 27z 27x.}

Razvan Gelca.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We are given the system of equations:
{x3=3x−12y+50,y3=12y+3z−2,z3=27z+27x. \begin{cases} x^3 = 3x - 12y + 50, \\ y^3 = 12y + 3z - 2, \\ z^3 = 27z + 27x. \end{cases}

To find all triples (x,y,z)(x, y, z) of real numbers that satisfy these equations, we analyze the behavior of the functions involved.

1. **Case x>2x > 2:**
- For x>2x > 2, the function x3−3xx^3 - 3x is increasing. Thus, from the first equation, y<4y < 4.
- From the third equation, since x>2x > 2, we have z3−27z>54z^3 - 27z > 54, implying z>6z > 6.
- However, substituting z>6z > 6 into the second equation gives y3−12y>16y^3 - 12y > 16, implying y>4y > 4, which contradicts y<4y < 4.

2. **Case x<2x < 2:**
- For x<2x < 2, from the third equation, z3−27z<54z^3 - 27z < 54, implying z<6z < 6.
- From the second equation, y<4y < 4.
- Substituting y<4y < 4 into the first equation gives x3−3x=50−12y>2x^3 - 3x = 50 - 12y > 2, implying x>2x > 2, which contradicts x<2x < 2.

3. **Verification of x=2,y=4,z=6x = 2, y = 4, z = 6:**
- Substituting x=2x = 2, y=4y = 4, and z=6z = 6 into the equations:
23=3(2)−12(4)+50  ⟹  8=6−48+50  ⟹  8=8, 2^3 = 3(2) - 12(4) + 50 \implies 8 = 6 - 48 + 50 \implies 8 = 8,
43=12(4)+3(6)−2  ⟹  64=48+18−2  ⟹  64=64, 4^3 = 12(4) + 3(6) - 2 \implies 64 = 48 + 18 - 2 \implies 64 = 64,
63=27(6)+27(2)  ⟹  216=162+54  ⟹  216=216. 6^3 = 27(6) + 27(2) \implies 216 = 162 + 54 \implies 216 = 216.
- These values satisfy all three equations.

Thus, the only solution is:
(x,y,z)=(2,4,6). (x, y, z) = \boxed{(2, 4, 6)}.

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