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

所有题目均经美国数学协会(MAA)官方合法授权使用。

15.

ω1\omega_1ω2\omega_2 交于点 XXYY。直线 \ell 分别在 AABB 处与 ω1\omega_1ω2\omega_2 相切,且直线 ABAB 离点 XX 比离点 YY 更近。圆 ω\omega 经过 AABB,并再次与 ω1\omega_1 交于 DAD \ne A,再次与 ω2\omega_2 交于 CBC \ne B。三点 CCYYDD 共线,XC=67XC = 67XY=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 \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 DAD \ne A and intersecting ω2\omega_2 again at CB.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
解答:

直线 ADADω\omegaω1\omega_1 的根轴,直线 BCBCω\omegaω2\omega_2 的根轴,直线 XYXYω1\omega_1ω2\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=XYABM = XY \cap ABMM 对每个圆的幂给出 MA2=MXMY=MB2MA^2 = MX \cdot MY = MB^2,所以 MMAB\overline{AB} 的中点,且 XXMMYY。 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 CCYYDD 共线,这两个角相加为 180180^\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 BYZABY \parallel ZA,对称地, AYZBAY \parallel ZB。因此 AYBZAYBZ 是平行四边形,并且由于 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)XZC=XAB=XYA\angle XZC = \angle XAB = \angle XYA =XDZ= \angle XDZ,所以三角形 XZCXZCXDZXDZ 相似,得到 XZ2=XCXDXZ^2 = XC \cdot XD

合并这些结论, AB2=4MA2=4MXMY=(MX+MY)2(MYMX)2=XZ2XY2=XCXDXY2, \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} 等于 6737472=2479220967 \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=XYAB.M = XY \cap AB. The power of MM with respect to each circle gives MA2=MXMY=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 BYZA,BY \parallel ZA, and symmetrically AYZB.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=XCXD.XZ^2 = XC \cdot XD.

Putting it together, AB2=4MA2=4MXMY=(MX+MY)2(MYMX)2=XZ2XY2=XCXDXY2, \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 6737472=2479220967 \cdot 37 - 47^2 = 2479 - 2209 =270.= 270.

← 第 14 题#14
完整试卷

其他年份的第 15 题