2020 AIME I 第 13 题

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13.

DD 位于 ABC\triangle ABC 的边 BC\overline{BC} 上,且 AD\overline{AD} 平分 BAC\angle BACAD\overline{AD} 的垂直平分线分别与 ABC\angle ABCACB\angle ACB 的角平分线 交于点 EEFF,已知 AB=4AB = 4BC=5BC = 5CA=6CA = 6AEF\triangle AEF 的面积可写成 mnp\frac{m\sqrt{n}}{p},其中 mmpp 是互质正整数,且 nn 是不被任何质数平方整除的正整数。 求 m+n+pm + n + p

Point DD lies on side BC\overline{BC} of ABC\triangle ABC so that AD\overline{AD} bisects BAC.\angle BAC. The perpendicular bisector of AD\overline{AD} intersects the bisectors of ABC\angle ABC and ACB\angle ACB in points EE and F,F, respectively. Given that AB=4,AB = 4, BC=5,BC = 5, and CA=6,CA = 6, the area of AEF\triangle AEF can be written as mnp,\frac{m\sqrt{n}}{p}, where mm and pp are relatively prime positive integers, and nn is a positive integer not divisible by the square of any prime. Find m+n+p.m + n + p.

答案:36
知识点:角平分线圆周角三角学
难度评级:3160
解答:

在三角形 ABDABD 中,BB 点的内角平分线再次交 ABDABD 的外接圆于不含 BB 的弧 ADAD 的中点, 而这个弧中点位于 AD\overline{AD} 的垂直平分线上,所以 EE 正是这个弧中点。圆周角 EAD\angle EADEBD\angle EBD 对同一段弧 EDED,所以 EAD=B2\angle EAD = \frac{B}{2}。 同理 FAD=C2\angle FAD = \frac{C}{2},且 E,FE, F 位于直线 ADAD 的两侧。

MMAD\overline{AD} 的中点。在直角三角形 AMEAMEAMFAMF 中, ME=AMtanB2ME = AM\tan\frac{B}{2}MF=AMtanC2MF = AM\tan\frac{C}{2},所以 EF=AM(tanB2+tanC2)EF = AM\left(\tan\frac{B}{2} + \tan\frac{C}{2}\right),而 AA 到直线 EFEF 的距离为 AMAM。因此 [AEF][AEF] =12AM2(tanB2+tanC2)= \frac{1}{2}AM^2\left(\tan\frac{B}{2} + \tan\frac{C}{2}\right)

此处 BD=2BD = 2DC=3DC = 3,所以 AD2=ABACBDDCAD^2 = AB \cdot AC - BD \cdot DC =246= 24 - 6 =18= 18, 且 AM2=92AM^2 = \frac{9}{2}。余弦定理给出 cosB=18\cos B = \frac{1}{8}cosC=34\cos C = \frac{3}{4},所以 tanB2=11/81+1/8=73\tan\frac{B}{2} = \sqrt{\frac{1 - 1/8}{1 + 1/8}} = \frac{\sqrt{7}}{3},并且 tanC2=17\tan\frac{C}{2} = \frac{1}{\sqrt{7}},二者之和为 10721\frac{10\sqrt{7}}{21} 面积为 129210721=15714\frac{1}{2} \cdot \frac{9}{2} \cdot \frac{10\sqrt{7}}{21} = \frac{15\sqrt{7}}{14},所以 m+n+p=15+7+14=36m + n + p = 15 + 7 + 14 = 36

In triangle ABD,ABD, the internal bisector of the angle at BB meets the circumcircle of ABDABD again at the midpoint of arc ADAD not containing B,B, and that arc midpoint lies on the perpendicular bisector of AD\overline{AD} — so EE is exactly that arc midpoint. The inscribed angles EAD\angle EAD and EBD\angle EBD subtend the same arc ED,ED, so EAD=B2.\angle EAD = \frac{B}{2}. Similarly FAD=C2,\angle FAD = \frac{C}{2}, and E,FE, F lie on opposite sides of line AD.AD.

Let MM be the midpoint of AD.\overline{AD}. In right triangles AMEAME and AMF,AMF, ME=AMtanB2ME = AM\tan\frac{B}{2} and MF=AMtanC2,MF = AM\tan\frac{C}{2}, so EF=AM(tanB2+tanC2),EF = AM\left(\tan\frac{B}{2} + \tan\frac{C}{2}\right), while the distance from AA to line EFEF is AM.AM. Hence [AEF][AEF] =12AM2(tanB2+tanC2).= \frac{1}{2}AM^2\left(\tan\frac{B}{2} + \tan\frac{C}{2}\right).

Here BD=2BD = 2 and DC=3,DC = 3, so AD2=ABACBDDCAD^2 = AB \cdot AC - BD \cdot DC =246= 24 - 6 =18= 18 and AM2=92.AM^2 = \frac{9}{2}. The law of cosines gives cosB=18\cos B = \frac{1}{8} and cosC=34,\cos C = \frac{3}{4}, so tanB2=11/81+1/8=73\tan\frac{B}{2} = \sqrt{\frac{1 - 1/8}{1 + 1/8}} = \frac{\sqrt{7}}{3} and tanC2=17,\tan\frac{C}{2} = \frac{1}{\sqrt{7}}, with sum 10721.\frac{10\sqrt{7}}{21}. The area is 129210721=15714,\frac{1}{2} \cdot \frac{9}{2} \cdot \frac{10\sqrt{7}}{21} = \frac{15\sqrt{7}}{14}, so m+n+p=15+7+14=36.m + n + p = 15 + 7 + 14 = 36.

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