2023 AIME I 第 12 题

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12.

设 △ABC\triangle ABC 是边长为 5555 的等边三角形。点 DD、EE、FF 分别在 BC‾\overline{BC}、CA‾\overline{CA}、AB‾\overline{AB} 上,且 BD=7BD = 7、CE=30CE = 30、AF=40AF = 40。点 PP 在 △ABC\triangle ABC 内部,并满足 ∠AEP=∠BFP=∠CDP。\angle AEP = \angle BFP = \angle CDP\text{。} 求 tan⁡2(∠AEP)\tan^2(\angle AEP)。

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 tan⁡2(∠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
小提示:

把从 EE、FF、DD 到 PP 的每个向量分解成沿边方向的分量和到该边的距离;沿边分量等于距离乘以 cot⁡θ\cot\theta。

Decompose each vector from E,E, F,F, DD to PP into a part along the side and the distance to that side; the along-part equals the distance times cot⁡θ.\cot\theta.

大提示:

把三个关系相加:方向 C→AC \to A、A→BA \to B、B→CB \to C 的单位向量和为零,而三个距离之和等于高(维维亚尼定理)。

Add the three relations: the unit directions C→A,C \to A, A→B,A \to B, B→CB \to C sum to zero, and the three distances sum to the height (Viviani).

解答:

设 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 C→AC \to A、A→BA \to B、B→CB \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 P−EP - 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 (P−E)⋅u1=d1cot⁡θ(P - E)\cdot\mathbf{u}_1 = d_1\cot\theta;同理 (P−F)⋅u2=d2cot⁡θ(P - F)\cdot\mathbf{u}_2 = d_2\cot\theta,且 (P−D)⋅u3=d3cot⁡θ(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 E⋅u1=52E \cdot \mathbf{u}_1 = \frac{5}{2}、F⋅u2=−15F \cdot \mathbf{u}_2 = -15,且 D⋅u3=7D \cdot \mathbf{u}_3 = 7,所以 5532 cot⁡θ=−(52−15+7)=112。 \begin{aligned} \frac{55\sqrt{3}}{2}\,\cot\theta &= -\left(\frac{5}{2} - 15 + 7\right) \\ &= \frac{11}{2} \end{aligned}\text{。}

因此 cot⁡θ=11553=153\cot\theta = \frac{11}{55\sqrt{3}} = \frac{1}{5\sqrt{3}},所以 tan⁡2(∠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 C→A,C \to A, A→B,A \to B, B→CB \to C — the directions from EE toward A,A, from FF toward B,B, and from DD toward C.C. Splitting P−EP - 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 (P−E)⋅u1=d1cot⁡θ;(P - E)\cdot\mathbf{u}_1 = d_1\cot\theta; similarly (P−F)⋅u2=d2cot⁡θ(P - F)\cdot\mathbf{u}_2 = d_2\cot\theta and (P−D)⋅u3=d3cot⁡θ.(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 E⋅u1=52,E \cdot \mathbf{u}_1 = \frac{5}{2}, F⋅u2=−15,F \cdot \mathbf{u}_2 = -15, and D⋅u3=7,D \cdot \mathbf{u}_3 = 7, so 5532 cot⁡θ=−(52−15+7)=112. \begin{aligned} \frac{55\sqrt{3}}{2}\,\cot\theta &= -\left(\frac{5}{2} - 15 + 7\right) \\ &= \frac{11}{2}. \end{aligned}

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

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