2005 AIME II Problem 15

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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.

Answer: 169
Concepts:ellipsetangent circlescompleting the squarequadratic
Difficulty rating: 3160
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.

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