2020 AIME I 第 15 题

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15.

ABC\triangle ABC 为锐角三角形,外接圆为 ω\omega,垂心为 HH。设 HBC\triangle HBC 的外接圆 在 HH 处的切线与 ω\omega 交于点 XXYY,且 HA=3HA = 3HX=2HX = 2HY=6HY = 6ABC\triangle ABC 的面积可写成 mnm\sqrt{n},其中 mmnn 是正整数,且 nn 不被任何质数的平方整除。 求 m+nm + n

Let ABC\triangle ABC be an acute triangle with circumcircle ω\omega and orthocenter H.H. Suppose the tangent to the circumcircle of HBC\triangle HBC at HH intersects ω\omega at points XX and YY with HA=3,HA = 3, HX=2,HX = 2, and HY=6.HY = 6. The area of ABC\triangle ABC can be written as mn,m\sqrt{n}, where mm and nn are positive integers, and nn is not divisible by the square of any prime. Find m+n.m + n.

答案:58
知识点:外接圆、外心与外接圆半径变换向量
难度评级:3500
解答:

HH 关于直线 BCBC 反射会落在 ω\omega 上,所以三角形 HBCHBC 的外接圆是 ω\omega 关于 BCBC 的反射。取外心 OO 为原点,则向量满足 H=A+B+CH = A + B + C。若 MMBC\overline{BC} 的中点,则 OMBCOM \perp BC,所以反射后的圆心是 2MO=B+C=HA2M - O = B + C = H - A。在 HH 处相切意味着 XYXY 垂直于从 B+CB + CHH 的半径,该半径就是向量 AA:弦 XYXY 垂直于 OAOA

放置 A=(0,R)A = (0, R),使 XYXY 是高度为 hh 的水平线,并令 H=(x0,h)H = (x_0, h)。 半弦长为 R2h2\sqrt{R^2 - h^2},而 HX=2HX = 2HY=6HY = 6 给出 R2h2=4\sqrt{R^2 - h^2} = 4,且 x0=2|x_0| = 2。由 HA=3HA = 34+(Rh)2=94 + (R - h)^2 = 9,所以 Rh=5R - h = \sqrt{5}。于是 16=R2h2=(Rh)(R+h)=5(2R5), \begin{aligned} 16 &= R^2 - h^2 \\ &= (R - h)(R + h) \\ &= \sqrt{5}\left(2R - \sqrt{5}\right), \end{aligned} R=2125R = \frac{21}{2\sqrt{5}}

现在 B+C=HA=(±2,5)B + C = H - A = (\pm 2, -\sqrt{5}),所以 M=(±1,52)M = \left(\pm 1, -\frac{\sqrt{5}}{2}\right),且 OM=32OM = \frac{3}{2},于是 BC=2R294=2995BC = 2\sqrt{R^2 - \frac{9}{4}} = 2\sqrt{\frac{99}{5}}AA 到直线 BCBC (过 MM, 且垂直于 OMOM)的距离为 AMOM2OM=21/4+9/43/2=5\frac{|A \cdot M - OM^2|}{OM} = \frac{21/4 + 9/4}{3/2} = 5,这里使用了 AM=5R2=214A \cdot M = -\frac{\sqrt{5}R}{2} = -\frac{21}{4}。因此 [ABC]=1229955=495=355, \begin{aligned} [ABC] &= \frac{1}{2} \cdot 2\sqrt{\frac{99}{5}} \cdot 5 \\ &= \sqrt{495} \\ &= 3\sqrt{55}, \end{aligned} 所以 m+n=3+55=58m + n = 3 + 55 = 58

Reflecting HH over line BCBC lands on ω,\omega, so the circumcircle of HBCHBC is the reflection of ω\omega over BC.BC. Take the circumcenter OO as the origin, so that H=A+B+CH = A + B + C as vectors. If MM is the midpoint of BC,\overline{BC}, then OMBC,OM \perp BC, so the reflected center is 2MO=B+C=HA.2M - O = B + C = H - A. Tangency at HH means XYXY is perpendicular to the radius from B+CB + C to H,H, which is the vector A:A: the chord XYXY is perpendicular to OA.OA.

Place A=(0,R)A = (0, R) so that XYXY is horizontal at height h,h, with H=(x0,h).H = (x_0, h). The half-chord length is R2h2,\sqrt{R^2 - h^2}, and HX=2,HX = 2, HY=6HY = 6 give R2h2=4\sqrt{R^2 - h^2} = 4 with x0=2.|x_0| = 2. From HA=3:HA = 3: 4+(Rh)2=9,4 + (R - h)^2 = 9, so Rh=5.R - h = \sqrt{5}. Then 16=R2h2=(Rh)(R+h)=5(2R5), \begin{aligned} 16 &= R^2 - h^2 \\ &= (R - h)(R + h) \\ &= \sqrt{5}\left(2R - \sqrt{5}\right), \end{aligned} giving R=2125.R = \frac{21}{2\sqrt{5}}.

Now B+C=HA=(±2,5),B + C = H - A = (\pm 2, -\sqrt{5}), so M=(±1,52)M = \left(\pm 1, -\frac{\sqrt{5}}{2}\right) and OM=32,OM = \frac{3}{2}, whence BC=2R294=2995.BC = 2\sqrt{R^2 - \frac{9}{4}} = 2\sqrt{\frac{99}{5}}. The distance from AA to line BCBC (through M,M, perpendicular to OMOM) is AMOM2OM=21/4+9/43/2=5,\frac{|A \cdot M - OM^2|}{OM} = \frac{21/4 + 9/4}{3/2} = 5, using AM=5R2=214.A \cdot M = -\frac{\sqrt{5}R}{2} = -\frac{21}{4}. Hence [ABC]=1229955=495=355, \begin{aligned} [ABC] &= \frac{1}{2} \cdot 2\sqrt{\frac{99}{5}} \cdot 5 \\ &= \sqrt{495} \\ &= 3\sqrt{55}, \end{aligned} and m+n=3+55=58.m + n = 3 + 55 = 58.

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