2005 AIME II Problem 15

Below is the professionally curated solution for Problem 15 of the 2005 AIME II, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2005 AIME II solutions, or check the answer key.

All of the real AMC 8, AMC 10, AMC 12, and AIME problems in our complete solution collection are used with official legal permission of the Mathematical Association of America (MAA).

Concepts:ellipsetangent circlescompleting the squarequadratic

Difficulty rating: 3160

15.

Let ω1\omega_1 and ω2\omega_2 denote the circles x2+y2+10x24y87=0x^2 + y^2 + 10x - 24y - 87 = 0 and x2+y210x24y+153=0,x^2 + y^2 - 10x - 24y + 153 = 0, respectively. Let mm be the smallest positive value of aa for which the line y=axy = ax contains the center of a circle that is internally tangent to ω1\omega_1 and externally tangent to ω2.\omega_2. Given that m2=pq,m^2 = \frac{p}{q}, where pp and qq are relatively prime positive integers, find p+q.p + q.

Solution:

Completing the square gives ω1 ⁣:(x+5)2+(y12)2=256\omega_1\colon (x+5)^2 + (y-12)^2 = 256 and ω2 ⁣:(x5)2+(y12)2=16,\omega_2\colon (x-5)^2 + (y-12)^2 = 16, with centers F1=(5,12)F_1 = (-5, 12) and F2=(5,12)F_2 = (5, 12) and radii 1616 and 4.4. If a circle with center PP and radius rr is internally tangent to ω1\omega_1 and externally tangent to ω2,\omega_2, then PF1=16rPF_1 = 16 - r and PF2=4+r,PF_2 = 4 + r, so PF1+PF2=20.PF_1 + PF_2 = 20.

Thus PP lies on the ellipse with foci F1F_1 and F2F_2 and major axis 20:20: the semimajor axis is 10,10, the center-to-focus distance is 5,5, so the semiminor axis squared is 10025=75,100 - 25 = 75, giving x2100+(y12)275=1,\frac{x^2}{100} + \frac{(y - 12)^2}{75} = 1, i.e. 3x2+4y296y+576=300.3x^2 + 4y^2 - 96y + 576 = 300. Substituting y=axy = ax yields (3+4a2)x296ax+276=0.(3 + 4a^2)x^2 - 96ax + 276 = 0.

The line y=axy = ax contains such a center exactly when this quadratic has a real root, i.e. (96a)24276(3+4a2)0,(96a)^2 - 4 \cdot 276\,(3 + 4a^2) \ge 0, which simplifies to 4800a23312,4800a^2 \ge 3312, so a269100.a^2 \ge \frac{69}{100}. The smallest positive such aa has m2=69100,m^2 = \frac{69}{100}, and p+q=69+100=169.p + q = 69 + 100 = 169.

← Problem 14Full Exam

Problem 15 in Other Years