Maths Olympiad Prep

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Combinatorics Difficulty 5.6 AIME, harder Prove it Bulgaria

Problem:

Aleksander writes a positive integer as a coefficient of a polynomial of degree four, then Elitza writes a positive integer as another coefficient of the same polynomial and so on till all the five coefficients of the polynomial are filled in. Aleksander wins if the polynomial obtained has an integer root; otherwise, Elitza wins. Who of them has a winning strategy?

Solution

Solution:

We shall prove that Elitza has a winning strategy. If the polynomial is a0x4+a1x3+a2x2+a3x+a4a_{0} x^{4} + a_{1} x^{3} + a_{2} x^{2} + a_{3} x + a_{4} and Aleksander writes a0a_{0}, a1a_{1}, a2a_{2} or a3a_{3}, then Elitza writes respectively a1=a0a_{1} = a_{0}, a0=a1a_{0} = a_{1}, a3=a2a_{3} = a_{2} or a2=a3a_{2} = a_{3}; if he writes a4a_{4}, she writes a1=1a_{1} = 1.

In a similar way Elitza is able to get a1a0a_{1} \leq a_{0} and a3a2a_{3} \leq a_{2} after her second move. Suppose that the polynomial obtained has an integer root y-y. Then y1y \geq 1 and hence a4=y3(a1a0y)+a3a2y0a_{4} = y^{3}(a_{1} - a_{0} y) + a_{3} - a_{2} y \leq 0, which is a contradiction.

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