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HMMT February

Algebra Difficulty 5.2 AIME, harder Find the answer

Let a,b,ca, b, c be integers. Define f(x)=ax2+bx+cf(x)=a x^{2}+b x+c. Suppose there exist pairwise distinct integers u,v,wu, v, w such that f(u)=0,f(v)=0f(u)=0, f(v)=0, and f(w)=2f(w)=2. Find the maximum possible value of the discriminant b2−4acb^{2}-4 a c of ff.

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

Solution

By the factor theorem, f(x)=a(x−u)(x−v)f(x)=a(x-u)(x-v), so the constraints essentially boil down to 2=f(w)=a(w−u)(w−v)2=f(w)=a(w-u)(w-v). We want to maximize the discriminant b2−4ac=a2[(u+v)2−4uv]=a2(u−v)2=a2[(w−v)−(w−u)]2b^{2}-4 a c=a^{2}\left[(u+v)^{2}-4 u v\right]=a^{2}(u-v)^{2}=a^{2}[(w-v)-(w-u)]^{2}. Clearly a∣2a \mid 2. If a>0a>0, then (w−u)(w−v)=2/a>0(w-u)(w-v)=2 / a>0 means the difference ∣u−v∣|u-v| is less than 2/a2 / a, whereas if a<0a<0, since at least one of ∣w−u∣|w-u| and ∣w−v∣|w-v| equals 1, the difference ∣u−v∣|u-v| of factors is greater than 2/∣a∣2 /|a|. So the optimal choice occurs either for a=−1a=-1 and ∣u−v∣=3|u-v|=3, or a=−2a=-2 and ∣u−v∣=2|u-v|=2. The latter wins, giving a discriminant of (−2)2⋅22=16(-2)^{2} \cdot 2^{2}=16.

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