2016 AIME I 第 15 题

先试着解答 2016 AIME I 第 15 题,然后核对你的答案与精心整理的解答,解答来自 LIVE by Po-Shen Loh。你也可以参加完整限时模拟考试、查看全部 2016 AIME I 解答,或核对答案。

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15.

圆 ω1\omega_1 和 ω2\omega_2 交于点 XX 和 YY。直线 ℓ\ell 分别在 AA 和 BB 处与 ω1\omega_1 和 ω2\omega_2 相切,且直线 ABAB 离点 XX 比离点 YY 更近。圆 ω\omega 经过 AA 和 BB,并再次与 ω1\omega_1 交于 D≠AD \ne A,再次与 ω2\omega_2 交于 C≠BC \ne B。三点 CC、YY、DD 共线,XC=67XC = 67,XY=47XY = 47,且 XD=37XD = 37。求 AB2AB^2。

Circles ω1\omega_1 and ω2\omega_2 intersect at points XX and Y.Y. Line ℓ\ell is tangent to ω1\omega_1 and ω2\omega_2 at AA and B,B, respectively, with line ABAB closer to point XX than to Y.Y. Circle ω\omega passes through AA and BB intersecting ω1\omega_1 again at D≠AD \ne A and intersecting ω2\omega_2 again at C≠B.C \ne B. The three points C,C, Y,Y, DD are collinear, XC=67,XC = 67, XY=47,XY = 47, and XD=37.XD = 37. Find AB2.AB^2.

答案:270
知识点:根轴圆幂导角相似
难度评级:3700
小提示:

直线 ADAD、BCBC 和 XYXY 是三对圆的根轴,所以它们交于一点 ZZ

Lines AD,AD, BC,BC, and XYXY are radical axes of pairs of the circles, so they meet at a point ZZ

大提示:

若 M=XY∩ABM = XY \cap AB,则 MA2=MX⋅MY=MB2MA^2 = MX \cdot MY = MB^2;证明 AYBZAYBZ 是平行四边形,并且 XZ2=XC⋅XDXZ^2 = XC \cdot XD

If M=XY∩AB,M = XY \cap AB, then MA2=MX⋅MY=MB2;MA^2 = MX \cdot MY = MB^2; show AYBZAYBZ is a parallelogram and XZ2=XC⋅XDXZ^2 = XC \cdot XD

解答:

直线 ADAD 是 ω\omega 和 ω1\omega_1 的根轴,直线 BCBC 是 ω\omega 和 ω2\omega_2 的根轴,直线 XYXY 是 ω1\omega_1 和 ω2\omega_2 的根轴,所以三条直线交于根心 ZZ。(它们不能平行:否则会迫使构型对称并得到 XC=XDXC = XD。)令 M=XY∩ABM = XY \cap AB。点 MM 对每个圆的幂给出 MA2=MX⋅MY=MB2MA^2 = MX \cdot MY = MB^2,所以 MM 是 AB‾\overline{AB} 的中点,且 XX 位于 MM 与 YY 之间。

因为 ADYXADYX 共圆,∠XAZ=∠XYD\angle XAZ = \angle XYD;因为 BCYXBCYX 共圆,∠XBZ=∠XYC\angle XBZ = \angle XYC。由于 CC、YY、DD 共线,这两个角之和为 180∘180^\circ,所以 ZAXBZAXB 共圆。点 BB 处的切线弦角给出 ∠XYB=∠ABX=∠AZX\angle XYB = \angle ABX = \angle AZX,所以 BY∥ZABY \parallel ZA;同理,AY∥ZBAY \parallel ZB。因此 AYBZAYBZ 是平行四边形。由于 MM 是对角线 AB‾\overline{AB} 的中点,它也是 ZY‾\overline{ZY} 的中点;因此 XZ=XM+MZXZ = XM + MZ =MX+MY= MX + MY。另外,∠XCZ=∠XYB=∠XZD\angle XCZ = \angle XYB = \angle XZD,并且由点 AA 处的切线弦角可得 ∠XZC=∠XAB=∠XYA\angle XZC = \angle XAB = \angle XYA =∠XDZ= \angle XDZ,所以三角形 XZCXZC 和 XDZXDZ 相似,得到 XZ2=XC⋅XDXZ^2 = XC \cdot XD。

合并这些结论,AB2=4MA2=4 MX⋅MY=(MX+MY)2−(MY−MX)2=XZ2−XY2=XC⋅XD−XY2, \begin{aligned} AB^2 &= 4MA^2 = 4\,MX \cdot MY \\ &= (MX + MY)^2 \\ &\quad {}- (MY - MX)^2 \\ &= XZ^2 - XY^2 \\ &= XC \cdot XD - XY^2 \end{aligned}\text{,}等于 67⋅37−472=2479−220967 \cdot 37 - 47^2 = 2479 - 2209 =270= 270。

Line ADAD is the radical axis of ω\omega and ω1,\omega_1, line BCBC that of ω\omega and ω2,\omega_2, and line XYXY that of ω1\omega_1 and ω2,\omega_2, so the three lines meet at the radical center Z.Z. (They cannot be parallel: that would force a symmetric configuration with XC=XD.XC = XD.) Let M=XY∩AB.M = XY \cap AB. The power of MM with respect to each circle gives MA2=MX⋅MY=MB2,MA^2 = MX \cdot MY = MB^2, so MM is the midpoint of AB‾,\overline{AB}, with XX between MM and Y.Y.

Since ADYXADYX is cyclic, ∠XAZ=∠XYD,\angle XAZ = \angle XYD, and since BCYXBCYX is cyclic, ∠XBZ=∠XYC;\angle XBZ = \angle XYC; as C,C, Y,Y, DD are collinear these add to 180∘,180^\circ, so ZAXBZAXB is cyclic. The tangent-chord angle at BB gives ∠XYB=∠ABX=∠AZX,\angle XYB = \angle ABX = \angle AZX, so BY∥ZA,BY \parallel ZA, and symmetrically AY∥ZB.AY \parallel ZB. Hence AYBZAYBZ is a parallelogram, and since MM is the midpoint of diagonal AB‾,\overline{AB}, it is also the midpoint of ZY‾:\overline{ZY}: therefore XZ=XM+MZXZ = XM + MZ =MX+MY.= MX + MY. Moreover ∠XCZ=∠XYB=∠XZD\angle XCZ = \angle XYB = \angle XZD and (by the tangent-chord angle at AA) ∠XZC=∠XAB=∠XYA\angle XZC = \angle XAB = \angle XYA =∠XDZ,= \angle XDZ, so triangles XZCXZC and XDZXDZ are similar, giving XZ2=XC⋅XD.XZ^2 = XC \cdot XD.

Putting it together, AB2=4MA2=4 MX⋅MY=(MX+MY)2−(MY−MX)2=XZ2−XY2=XC⋅XD−XY2, \begin{aligned} AB^2 &= 4MA^2 = 4\,MX \cdot MY \\ &= (MX + MY)^2 \\ &\quad {}- (MY - MX)^2 \\ &= XZ^2 - XY^2 \\ &= XC \cdot XD - XY^2, \end{aligned} which equals 67⋅37−472=2479−220967 \cdot 37 - 47^2 = 2479 - 2209 =270.= 270.

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