2000 AIME II Problem 13

Below is the professionally curated solution for Problem 13 of the 2000 AIME II, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2000 AIME II solutions, or check the answer key.

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Concepts:polynomialsum and difference of cubesfactoringquadratic

Difficulty rating: 2920

13.

The equation 2000x6+100x5+10x3+x2=02000x^6 + 100x^5 + 10x^3 + x - 2 = 0 has exactly two real roots, one of which is m+nr,\frac{m + \sqrt{n}}{r}, where m,m, n,n, and rr are integers, mm and rr are relatively prime, and r>0.r \gt 0. Find m+n+r.m + n + r.

Solution:

Group the equation as 2(1000x61)+x(100x4+10x2+1)=0.2(1000x^6 - 1) + x(100x^4 + 10x^2 + 1) = 0. Since 1000x61=(10x2)31=(10x21)(100x4+10x2+1),1000x^6 - 1 = (10x^2)^3 - 1 = (10x^2 - 1)(100x^4 + 10x^2 + 1), the left side factors as (100x4+10x2+1)(2(10x21)+x)=(100x4+10x2+1)(20x2+x2).(100x^4 + 10x^2 + 1)\big(2(10x^2 - 1) + x\big) = (100x^4 + 10x^2 + 1)(20x^2 + x - 2).

The quartic factor is always positive, so the two real roots are the roots of 20x2+x2=0,20x^2 + x - 2 = 0, namely x=1±16140.x = \frac{-1 \pm \sqrt{161}}{40}. The root of the form m+nr\frac{m + \sqrt{n}}{r} is 1+16140,\frac{-1 + \sqrt{161}}{40}, with m=1,m = -1, n=161,n = 161, r=40,r = 40, and gcd(1,40)=1.\gcd(-1, 40) = 1. Thus m+n+r=1+161+40=200.m + n + r = -1 + 161 + 40 = 200.

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