2019 AIME I Problem 15

Below is the professionally curated solution for Problem 15 of the 2019 AIME I, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2019 AIME I solutions, or check the answer key.

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Concepts:radical axispower of a pointtangent circles

Difficulty rating: 3500

15.

Let AB\overline{AB} be a chord of a circle ω,\omega, and let PP be a point on the chord AB.\overline{AB}. Circle ω1\omega_1 passes through AA and PP and is internally tangent to ω.\omega. Circle ω2\omega_2 passes through BB and PP and is internally tangent to ω.\omega. Circles ω1\omega_1 and ω2\omega_2 intersect at points PP and Q.Q. Line PQPQ intersects ω\omega at XX and Y.Y. Assume that AP=5,AP = 5, PB=3,PB = 3, XY=11,XY = 11, and PQ2=mn,PQ^2 = \frac{m}{n}, where mm and nn are relatively prime positive integers. Find m+n.m + n.

Solution:

Since AA lies on both ω\omega and ω1\omega_1 and internally tangent circles meet only at their point of tangency, ω1\omega_1 is tangent to ω\omega at A;A; likewise ω2\omega_2 is tangent at B.B. Let ZZ be the intersection of the tangent lines to ω\omega at AA and B.B. Each tangent line is also tangent to the corresponding inner circle, so the powers of ZZ with respect to ω1\omega_1 and ω2\omega_2 are ZA2ZA^2 and ZB2,ZB^2, which are equal. Hence ZZ lies on the radical axis PQ,PQ, and along the line through Z,X,P,Q,Y:Z, X, P, Q, Y: ZPZQ=ZA2=ZXZY,ZP \cdot ZQ = ZA^2 = ZX \cdot ZY, the last equality because ZAZA is tangent to ω.\omega.

Because ZA=ZB,ZA = ZB, the point ZZ lies on the perpendicular bisector of AB;\overline{AB}; if MM is the midpoint of AB,\overline{AB}, then ZA2ZP2=MA2MP2=4212=15.ZA^2 - ZP^2 = MA^2 - MP^2 = 4^2 - 1^2 = 15. Also the power of PP in ω\omega gives XPPY=APPB=15.XP \cdot PY = AP \cdot PB = 15. Set s=ZPs = ZP and u=ZX,u = ZX, so ZY=u+11.ZY = u + 11. The relations become u(u+11)=s2+15,(su)(u+11s)=15.u(u + 11) = s^2 + 15, \qquad (s - u)(u + 11 - s) = 15. Expanding the second and substituting the first yields u=s112+15s,u = s - \frac{11}{2} + \frac{15}{s}, and substituting back gives (s+15s)21214=s2+15,\left(s + \frac{15}{s}\right)^2 - \frac{121}{4} = s^2 + 15, so 225s2=614.\frac{225}{s^2} = \frac{61}{4}.

Finally ZQ=ZA2ZP=s+15s,ZQ = \frac{ZA^2}{ZP} = s + \frac{15}{s}, so PQ=ZQZP=15sPQ = ZQ - ZP = \frac{15}{s} and PQ2=225s2=614.PQ^2 = \frac{225}{s^2} = \frac{61}{4}. Therefore m+n=61+4=65.m + n = 61 + 4 = 65.

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