2011 AIME I Problem 15

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15.

For some integer m,m, the polynomial x32011x+mx^3 - 2011x + m has the three integer roots a,a, b,b, and c.c. Find a+b+c.|a| + |b| + |c|.

Answer: 98
Concepts:Vieta’s FormulasDiophantine Equationperfect square
Difficulty rating: 3270
Solution:

By Vieta's formulas, a+b+c=0a + b + c = 0 and ab+bc+ca=2011.ab + bc + ca = -2011. Negating all three roots replaces mm by m,-m, so we may assume two roots, say ab,a \ge b, are nonnegative. Substituting c=(a+b)c = -(a + b) into the second equation gives ab(a+b)2=2011,ab - (a + b)^2 = -2011, that is, a2+ab+b2=2011.a^2 + ab + b^2 = 2011.

Multiplying by 44 and completing the square, (2a+b)2+3b2=8044.(2a + b)^2 + 3b^2 = 8044. Since ab0,a \ge b \ge 0, we have 3b2a2+ab+b2=2011,3b^2 \le a^2 + ab + b^2 = 2011, so 0b25.0 \le b \le 25. Checking these values, 80443b28044 - 3b^2 is a perfect square only for b=10,b = 10, where 8044300=7744=882.8044 - 300 = 7744 = 88^2.

Then 2a+b=882a + b = 88 gives a=39,a = 39, and c=(a+b)=49.c = -(a + b) = -49. (Indeed 3910492=2011.39 \cdot 10 - 49^2 = -2011.) Therefore a+b+c=39+10+49|a| + |b| + |c| = 39 + 10 + 49 =98.= 98.

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