Maths Olympiad Prep

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Algebra Difficulty 5.6 AIME, harder Prove it United States

Problem:
Let S\mathcal{S} be the set of all nonconstant monic polynomials PP with integer coefficients satisfying P(3+2)=P(32)P\left(\sqrt{3} + \sqrt{2}\right) = P\left(\sqrt{3} - \sqrt{2}\right). If QQ is an element of S\mathcal{S} with minimal degree, compute the only possible value of Q(10)Q(0)Q(10) - Q(0).

Solution

Solution:
First, note that the polynomial x410x2+1x^{4} - 10x^{2} + 1 has both 3+2\sqrt{3} + \sqrt{2} and 32\sqrt{3} - \sqrt{2} as roots. It suffices to check whether a polynomial of degree at most 3 belongs in S\mathcal{S}. Suppose f(x)=ax3+bx2+cx+dSf(x) = a x^{3} + b x^{2} + c x + d \in \mathcal{S}. We compute

(3+2)3(32)3=222(\sqrt{3} + \sqrt{2})^{3} - (\sqrt{3} - \sqrt{2})^{3} = 22\sqrt{2}
(3+2)2(32)2=46(\sqrt{3} + \sqrt{2})^{2} - (\sqrt{3} - \sqrt{2})^{2} = 4\sqrt{6}
(3+2)1(32)1=22,(\sqrt{3} + \sqrt{2})^{1} - (\sqrt{3} - \sqrt{2})^{1} = 2\sqrt{2},

so we get that

f(3+2)f(32)=(222)a+(46)b+(22)c.f(\sqrt{3} + \sqrt{2}) - f(\sqrt{3} - \sqrt{2}) = (22\sqrt{2})a + (4\sqrt{6})b + (2\sqrt{2})c.

By resolving linear dependencies, it's clear that b=0b = 0 and c=11ac = -11a. It follows that if ff is not the zero polynomial, it must be cubic. It is then clear that f(x)=x311x+df(x) = x^{3} - 11x + d has minimal degree in S\mathcal{S}, and thus Q(10)Q(0)=f(10)f(0)=890Q(10) - Q(0) = f(10) - f(0) = \boxed{890}.

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