2023 AIME I 第 12 题

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12.

ABC\triangle ABC 是边长为 5555 的等边三角形。点 DDEEFF 分别在 BC\overline{BC}CA\overline{CA}AB\overline{AB} 上,且 BD=7BD = 7CE=30CE = 30AF=40AF = 40。点 PPABC\triangle ABC 内部,并满足求 tan2(AEP)\tan^2(\angle AEP)AEP=BFP=CDP.\angle AEP = \angle BFP = \angle CDP.

Let ABC\triangle ABC be an equilateral triangle with side length 55.55. Points D,D, E,E, and FF lie on BC,\overline{BC}, CA,\overline{CA}, and AB,\overline{AB}, respectively, with BD=7,BD = 7, CE=30,CE = 30, and AF=40.AF = 40. Point PP inside ABC\triangle ABC has the property that AEP=BFP=CDP.\angle AEP = \angle BFP = \angle CDP. Find tan2(AEP).\tan^2(\angle AEP).

答案:75
知识点:等边三角形维维亚尼定理向量三角学
难度评级:3270
解答:

B=(0,0)B = (0, 0)C=(55,0)C = (55, 0)A=(552,5532)A = \left(\frac{55}{2}, \frac{55\sqrt{3}}{2}\right), 则 D=(7,0)D = (7, 0)E=(40,153)E = (40, 15\sqrt{3}),且 F=(152,1532)F = \left(\frac{15}{2}, \frac{15\sqrt{3}}{2}\right)。令公共角为 θ\theta,并令 u1,u2,u3\mathbf{u}_1, \mathbf{u}_2, \mathbf{u}_3 分别为方向 CAC \to AABA \to BBCB \to C 的单位向量,也就是从 EE 指向 AA、从 FF 指向 BB、从 DD 指向 CC 的方向。把 PEP - E 分解为沿 u1\mathbf{u}_1 的分量和垂直于边 CACA 的分量,后者长度为 PP 到直线 CACA 的距离 d1d_1。角度条件给出 (PE)u1=d1cotθ(P - E)\cdot\mathbf{u}_1 = d_1\cot\theta;同理 (PF)u2=d2cotθ(P - F)\cdot\mathbf{u}_2 = d_2\cot\theta,且 (PD)u3=d3cotθ(P - D)\cdot\mathbf{u}_3 = d_3\cot\theta

现在把三个关系相加。因为 u1+u2+u3=0\mathbf{u}_1 + \mathbf{u}_2 + \mathbf{u}_3 = \mathbf{0} (三角形的有向边首尾相接),PP 项会消去;并且由 Viviani 定理, d1+d2+d3d_1 + d_2 + d_3 等于高 5532\frac{55\sqrt{3}}{2}。取 u1=(12,32)\mathbf{u}_1 = \left(-\frac{1}{2}, \frac{\sqrt{3}}{2}\right)u2=(12,32)\mathbf{u}_2 = \left(-\frac{1}{2}, -\frac{\sqrt{3}}{2}\right)、以及 u3=(1,0)\mathbf{u}_3 = (1, 0) 可得 Eu1=52E \cdot \mathbf{u}_1 = \frac{5}{2}Fu2=15F \cdot \mathbf{u}_2 = -15,且 Du3=7D \cdot \mathbf{u}_3 = 7,所以 5532cotθ=(5215+7)=112. \begin{aligned} \frac{55\sqrt{3}}{2}\,\cot\theta &= -\left(\frac{5}{2} - 15 + 7\right) \\ &= \frac{11}{2}. \end{aligned}

因此 cotθ=11553=153\cot\theta = \frac{11}{55\sqrt{3}} = \frac{1}{5\sqrt{3}},所以 tan2(AEP)=(53)2=75\tan^2(\angle AEP) = \left(5\sqrt{3}\right)^2 = 75

Place B=(0,0),B = (0, 0), C=(55,0),C = (55, 0), A=(552,5532),A = \left(\frac{55}{2}, \frac{55\sqrt{3}}{2}\right), so that D=(7,0),D = (7, 0), E=(40,153),E = (40, 15\sqrt{3}), and F=(152,1532).F = \left(\frac{15}{2}, \frac{15\sqrt{3}}{2}\right). Let θ\theta be the common angle and let u1,u2,u3\mathbf{u}_1, \mathbf{u}_2, \mathbf{u}_3 be the unit vectors in the directions CA,C \to A, AB,A \to B, BCB \to C — the directions from EE toward A,A, from FF toward B,B, and from DD toward C.C. Splitting PEP - E into its component along u1\mathbf{u}_1 and its component perpendicular to side CA,CA, whose length is the distance d1d_1 from PP to line CA,CA, the angle condition gives (PE)u1=d1cotθ;(P - E)\cdot\mathbf{u}_1 = d_1\cot\theta; similarly (PF)u2=d2cotθ(P - F)\cdot\mathbf{u}_2 = d_2\cot\theta and (PD)u3=d3cotθ.(P - D)\cdot\mathbf{u}_3 = d_3\cot\theta.

Now add all three relations. Since u1+u2+u3=0\mathbf{u}_1 + \mathbf{u}_2 + \mathbf{u}_3 = \mathbf{0} (the directed sides of a triangle close up), PP drops out, and by Viviani's theorem d1+d2+d3d_1 + d_2 + d_3 equals the height 5532.\frac{55\sqrt{3}}{2}. With u1=(12,32),\mathbf{u}_1 = \left(-\frac{1}{2}, \frac{\sqrt{3}}{2}\right), u2=(12,32),\mathbf{u}_2 = \left(-\frac{1}{2}, -\frac{\sqrt{3}}{2}\right), and u3=(1,0),\mathbf{u}_3 = (1, 0), we get Eu1=52,E \cdot \mathbf{u}_1 = \frac{5}{2}, Fu2=15,F \cdot \mathbf{u}_2 = -15, and Du3=7,D \cdot \mathbf{u}_3 = 7, so 5532cotθ=(5215+7)=112. \begin{aligned} \frac{55\sqrt{3}}{2}\,\cot\theta &= -\left(\frac{5}{2} - 15 + 7\right) \\ &= \frac{11}{2}. \end{aligned}

Hence cotθ=11553=153,\cot\theta = \frac{11}{55\sqrt{3}} = \frac{1}{5\sqrt{3}}, so tan2(AEP)=(53)2=75.\tan^2(\angle AEP) = \left(5\sqrt{3}\right)^2 = 75.

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