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Algebra Difficulty 6.2 National olympiad Prove it Vietnam

Find all polynomials P(x)P(x), Q(x)Q(x) with real coefficients, such that for each real number aa, P(a)P(a) is the solution of the following equation
x2023+Q(a)x2+(a2024+a)x+a3+2025a=0. x^{2023} + Q(a)x^2 + (a^{2024} + a)x + a^3 + 2025a = 0.

Solution

From the hypothesis, we have
P(x)2023+Q(x)P(x)2+(x2024+x)P(x)+x(x2+2025)=0,(1) P(x)^{2023} + Q(x)P(x)^2 + (x^{2024} + x)P(x) + x(x^2 + 2025) = 0, \quad (1)
for all xRx \in \mathbb{R}. From that, we deduce that the polynomial x(x2+2025)x(x^2 + 2025) is divisible by the polynomial P(x)P(x). Thus, the polynomial P(x)P(x) must have one of the following forms kk, kxkx, k(x2+2025)k(x^2 + 2025) or kx(x2+2025)kx(x^2 + 2025), where kk is some real constant. However, if polynomial P(x)P(x) is divisible by polynomial xx, then from equation (1), we deduce that the polynomial 2025x2025x is divisible by x2x^2, contradiction. Therefore, the polynomial P(x)P(x) must have one of two forms kk or k(x2+2025)k(x^2 + 2025) where kk is some non-zero constant.

* If P(x)=kP(x) = k then from equation (1), we have
k2023+k2Q(x)+k(x2024+x)+x3+2025x=0, k^{2023} + k^2 Q(x) + k(x^{2024} + x) + x^3 + 2025x = 0,
or
Q(x)=k(x2024+x)x32025xk2023k2 Q(x) = \frac{-k(x^{2024} + x) - x^3 - 2025x - k^{2023}}{k^2}
for all real numbers xx.

* If P(x)=k(x2+2025)P(x) = k(x^2 + 2025) then from equation (1), we have
k2023(x2+2025)2023+k2(x2+2025)2Q(x)+k(x2+2025)x(x2023+1)+x(x2+2025)=0, or k^{2023}(x^2 + 2025)^{2023} + k^2(x^2 + 2025)^2 Q(x) + k(x^2 + 2025)x(x^{2023} + 1) + x(x^2 + 2025) = 0, \text{ or}
k2023(x2+2025)2022+k(x2+2025)Q(x)+kx(x2023+1)+x=0, k^{2023}(x^2+2025)^{2022} + k(x^2+2025)Q(x) + kx(x^{2023}+1) + x = 0,
for all xRx \in \mathbb{R}. We obtain that (x2+2025)(kx(x2023+1)+x)(x^2+2025)(kx(x^{2023}+1)+x). On the other hand, the two polynomials xx and x2+2025x^2 + 2025 are coprime, so we deduce that (x2+2025)k(x2023+1)+1=kx2023+k+1(x^2 + 2025)|k(x^{2023} + 1) + 1 = kx^{2023} + k + 1. Putting c=2025c = -2025, we have x2c(modx2c)x^2 \equiv c \pmod{x^2 - c}, therefore
kx2023+k+1=k(x2)1011x+k+1kc1011x+k+1(modx2c). kx^{2023} + k + 1 = k(x^2)^{1011}x + k + 1 \\ \equiv kc^{1011}x + k + 1 \pmod{x^2 - c}.
Since (x2c)(kx2023+k+1)(x^2-c)|(kx^{2023}+k+1), we deduce that (x2c)(kc1011x+k+1)(x^2-c)|(kc^{1011}x+k+1). However, as deg(kc1011x+k+1)<deg(x2c)\deg(kc^{1011}x+k+1) < \deg(x^2-c), it follows that kc1011=0kc^{1011} = 0 and k+1=0k+1 = 0. But this system has no real solutions. Therefore, in this case there do not exist polynomials P(x),Q(x)P(x), Q(x) that satisfy the requirement.

In short, the polynomials satisfy the problem are of the form
P(x)=kP(x) = k, and
Q(x)=k(x2024+x)x32025xk2023k2, Q(x) = \frac{-k(x^{2024} + x) - x^3 - 2025x - k^{2023}}{k^2},
where kk is some non-zero number. \square

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