Maths Olympiad Prep

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, 2022

Algebra Difficulty 5.9 AIME, harder Prove it Bulgaria

Consider a quadratic polynomial f(x)f(x) with integer coefficients. If the values f(0)f(0), f(3)f(3), and f(4)f(4) are pairwise different numbers from the set {2,20,202,2022}\{2, 20, 202, 2022\}, then determine all possible values for f(1)f(1).

Solution

f(1){80,990}f(1) \in \{-80, -990\}.
Since fZ[x]f \in \mathbb{Z}[x], we have that yxf(y)f(x)y - x \mid f(y) - f(x), x,yZ\forall x, y \in \mathbb{Z}. Thus, 3f(3)f(0)3 \mid |f(3) - f(0)|, and 4f(4)f(0)4 \mid |f(4) - f(0)|. In the set {2,20,202,2022}\{2, 20, 202, 2022\}, only 22 and 2020 are congruent modulo 33, so {f(0),f(3)}={2,20}\{f(0), f(3)\} = \{2, 20\}. Modulo 44, 2020 has a different residue than the other three, so f(0)20f(0) \ne 20. We conclude that f(0)=2f(0) = 2, f(3)=20f(3) = 20, while f(4)=202f(4) = 202 or f(4)=2022f(4) = 2022.
Let f(x)=ax2+bx+cf(x) = ax^2 + bx + c. From c=f(0)=2c = f(0) = 2 and 9a+3b+2=f(3)=209a + 3b + 2 = f(3) = 20, we deduce that f(x)=ax2+(63a)x+2f(x) = ax^2 + (6 - 3a)x + 2. Moreover f(1)=82af(1) = 8 - 2a, so fZ[x]f \in \mathbb{Z}[x] whenever aZa \in \mathbb{Z}, due to
a=f(4)f(0)4f(3)f(0)3. a = \frac{f(4) - f(0)}{4} - \frac{f(3) - f(0)}{3}.
Case 1: f(4)=202f(4) = 202. Then a=202242023=2004183=506=44a = \frac{202 - 2}{4} - \frac{20 - 2}{3} = \frac{200}{4} - \frac{18}{3} = 50 - 6 = 44 and f(1)=82a=888=80f(1) = 8 - 2a = 8 - 88 = -80.

Case 2: f(4)=2022f(4) = 2022. Then a=2022242023=20204183=5056=499a = \frac{2022 - 2}{4} - \frac{20 - 2}{3} = \frac{2020}{4} - \frac{18}{3} = 505 - 6 = 499 and f(1)=82a=8998=990f(1) = 8 - 2a = 8 - 998 = -990.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.