2010 AIME II Problem 10

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Concepts:polynomialprime factorizationcasework

Difficulty rating: 2890

10.

Find the number of second-degree polynomials f(x)f(x) with integer coefficients and integer zeros for which f(0)=2010.f(0) = 2010.

Solution:

Write f(x)=a(xr)(xs)f(x) = a(x - r)(x - s) with integer roots r,s;r, s; such a polynomial is determined by aa and the unordered pair {r,s}.\{r, s\}. The condition f(0)=2010f(0) = 2010 says ars=2010=23567.a \cdot rs = 2010 = 2 \cdot 3 \cdot 5 \cdot 67. Since 20102010 is squarefree, each of the four primes goes entirely to one of a,|a|, r,|r|, s.|s|.

First suppose rs.|r| \ne |s|. Choosing which k1k \ge 1 of the four primes divide the roots ((4k)\binom{4}{k} ways) and splitting those primes between the two roots (2k12^{k-1} unordered ways) gives k=14(4k)2k1=4+12+16+8=40\sum_{k=1}^{4} \binom{4}{k} 2^{k-1} = 4 + 12 + 16 + 8 = 40 choices of magnitudes. For each, the four sign patterns (+,+),(+,+), (+,),(+,-), (,+),(-,+), (,)(-,-) of (r,s)(r, s) are distinct and each forces the sign of a,a, giving 440=1604 \cdot 40 = 160 polynomials.

If r=s,|r| = |s|, squarefreeness forces r=s=1,|r| = |s| = 1, so a=2010:|a| = 2010: the options are roots 1,11, 1 or 1,1-1, -1 with a=2010,a = 2010, or roots 1,11, -1 with a=2010,a = -2010, adding 33 more. In total 160+3=163.160 + 3 = 163.

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