2019 AIME I 第 15 题

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15.

AB\overline{AB} 为圆 ω\omega 的一条弦,点 PP 在弦 AB\overline{AB} 上。 圆 ω1\omega_1 经过 AAPP,并与 ω\omega 内切。圆 ω2\omega_2 经过 BBPP,并与 ω\omega 内切。圆 ω1\omega_1ω2\omega_2 交于点 PPQQ。直线 PQPQω\omega 交于 XXYY。已知 AP=5AP = 5PB=3PB = 3XY=11XY = 11,且 PQ2=mnPQ^2 = \frac{m}{n},其中 mmnn 是互质正整数。求 m+nm + n

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.

答案:65
知识点:根轴圆幂相切圆
难度评级:3500
解答:

因为 AA 同时在 ω\omegaω1\omega_1 上,而内切圆只在切点处相交,所以 ω1\omega_1AA; 处与 ω\omega 相切;同理 ω2\omega_2BB 处相切。设 ZZω\omegaAABB 处的切线交点。每条切线也分别是相应内圆的切线,所以 ZZ 关于 ω1\omega_1ω2\omega_2 的幂分别为 ZA2ZA^2ZB2ZB^2,二者相等。因此 ZZ 在根轴 PQPQ 上,并且沿过 Z,X,P,Q,YZ, X, P, Q, Y: 的直线有 最后一个等式来自 ZAZAω\omega 的切线。 ZPZQ=ZA2=ZXZY,ZP \cdot ZQ = ZA^2 = ZX \cdot ZY,

因为 ZA=ZBZA = ZBZZAB\overline{AB}; 的垂直平分线上;若 MMAB\overline{AB} 的中点,则 ZA2ZP2ZA^2 - ZP^2 =MA2MP2= MA^2 - MP^2 =4212=15= 4^2 - 1^2 = 15。 同时 PP 关于 ω\omega 的幂给出 XPPY=APPB=15XP \cdot PY = AP \cdot PB = 15。设 s=ZPs = ZPu=ZXu = ZX,于是 ZY=u+11ZY = u + 11。关系变为 展开第二式并代入第一式,得 u=s112+15su = s - \frac{11}{2} + \frac{15}{s},再代回可得 (s+15s)21214=s2+15\left(s + \frac{15}{s}\right)^2 - \frac{121}{4} = s^2 + 15,所以 225s2=614\frac{225}{s^2} = \frac{61}{4}u(u+11)=s2+15,(su)(u+11s)=15. \begin{aligned} u(u + 11) &= s^2 + 15, \\ (s - u)(u + 11 - s) &= 15. \end{aligned}

最后 ZQ=ZA2ZP=s+15sZQ = \frac{ZA^2}{ZP} = s + \frac{15}{s},所以 PQ=ZQZP=15sPQ = ZQ - ZP = \frac{15}{s},并且 PQ2=225s2=614PQ^2 = \frac{225}{s^2} = \frac{61}{4}。因此 m+n=61+4=65m + n = 61 + 4 = 65

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 ZA2ZP2ZA^2 - ZP^2 =MA2MP2= MA^2 - MP^2 =4212=15.= 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. \begin{aligned} u(u + 11) &= s^2 + 15, \\ (s - u)(u + 11 - s) &= 15. \end{aligned} 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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