2014 AIME I 第 13 题

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13.

在正方形 ABCDABCD 中,点 E,F,GE, F, GHH 分别在边 AB\overline{AB}BC\overline{BC}CD\overline{CD}DA\overline{DA} 上,且 EGFH\overline{EG} \perp \overline{FH},并且 EG=FH=34EG = FH = 34。线段 EG\overline{EG}FH\overline{FH} 交于点 PP,四边形 AEPHAEPHBFPEBFPECGPFCGPFDHPGDHPG 的面积之比为 269:275:405:411269 : 275 : 405 : 411。求正方形 ABCDABCD 的面积。

On square ABCD,ABCD, points E,F,G,E, F, G, and HH lie on sides AB,\overline{AB}, BC,\overline{BC}, CD,\overline{CD}, and DA,\overline{DA}, respectively, so that EGFH\overline{EG} \perp \overline{FH} and EG=FH=34.EG = FH = 34. Segments EG\overline{EG} and FH\overline{FH} intersect at a point P,P, and the areas of the quadrilaterals AEPH,AEPH, BFPE,BFPE, CGPF,CGPF, and DHPGDHPG are in the ratio 269:275:405:411.269 : 275 : 405 : 411. Find the area of square ABCD.ABCD.

答案:850
知识点:坐标几何正方形(几何)梯形方程组
难度评级:3500
解答:

B=(0,0)B = (0,0)C=(s,0)C = (s, 0)D=(s,s)D = (s, s)A=(0,s)A = (0, s),并设 E=(0,e)E = (0, e)F=(f,0)F = (f, 0)G=(s,g)G = (s, g)H=(h,s)H = (h, s)。区域 AEPHAEPHDHPGDHPG 合起来形成梯形 AEGDAEGD 面积为 s((se)+(sg))2\frac{s\,((s - e) + (s - g))}{2};由于 269+4111360=12\frac{269 + 411}{1360} = \frac{1}{2},这正好是 s2s^2 的一半,迫使 e+g=se + g = s, 即 EGEG 经过中心。同样,AEPHAEPHBFPEBFPE 形成梯形 ABFHABFH 面积为 s(f+h)2=269+2751360s2=25s2\frac{s(f + h)}{2} = \frac{269 + 275}{1360}\,s^2 = \frac{2}{5}s^2,所以 f+h=4s5f + h = \frac{4s}{5}。方向向量 (s,ge)(s, g - e)(hf,s)(h - f, s) 垂直,给出 hf=egh - f = e - g。令 δ=ge\delta = g - e,得到 E=(0,sδ2)E = \left(0, \frac{s - \delta}{2}\right)G=(s,s+δ2)G = \left(s, \frac{s + \delta}{2}\right)F=(2s5+δ2,0)F = \left(\frac{2s}{5} + \frac{\delta}{2}, 0\right)H=(2s5δ2,s)H = \left(\frac{2s}{5} - \frac{\delta}{2}, s\right),且 EG=34EG = 34 给出 s2+δ2=1156s^2 + \delta^2 = 1156

将直线 EGEGFHFH 相交,并用 s2+δ2=1156s^2 + \delta^2 = 1156 化简,可得 P=(s2s311560,  s2s2δ11560), \begin{aligned} P &= \\ &\left(\frac{s}{2} - \frac{s^3}{11560},\; \frac{s}{2} - \frac{s^2\delta}{11560}\right), \end{aligned} 再对 A,E,P,HA, E, P, H 使用鞋带公式,得到 [AEPH]=s25s3δ231200[AEPH] = \frac{s^2}{5} - \frac{s^3\delta}{231200}。令它等于 2691360s2\frac{269}{1360}s^2,得到 3s21360=s3δ231200\frac{3s^2}{1360} = \frac{s^3\delta}{231200},所以 sδ=510s\delta = 510

现在 s2+δ2=1156s^2 + \delta^2 = 1156,且 s2δ2=260100s^2\delta^2 = 260100,所以 s2s^2δ2\delta^2 是方程 t21156t+260100=0t^2 - 1156t + 260100 = 0 的两个根,分别为 1156±5442=850\frac{1156 \pm 544}{2} = 850306306。由于 δ=ge<s|\delta| = |g - e| \lt s,面积为 s2=850s^2 = 850

Place B=(0,0),B = (0,0), C=(s,0),C = (s, 0), D=(s,s),D = (s, s), A=(0,s),A = (0, s), with E=(0,e),E = (0, e), F=(f,0),F = (f, 0), G=(s,g),G = (s, g), H=(h,s).H = (h, s). The regions AEPHAEPH and DHPGDHPG together form the trapezoid AEGD,AEGD, of area s((se)+(sg))2;\frac{s\,((s - e) + (s - g))}{2}; since 269+4111360=12,\frac{269 + 411}{1360} = \frac{1}{2}, this is half of s2,s^2, forcing e+g=s,e + g = s, i.e. EGEG passes through the center. Likewise AEPHAEPH and BFPEBFPE form the trapezoid ABFHABFH of area s(f+h)2=269+2751360s2=25s2,\frac{s(f + h)}{2} = \frac{269 + 275}{1360}\,s^2 = \frac{2}{5}s^2, so f+h=4s5.f + h = \frac{4s}{5}. Perpendicularity of the directions (s,ge)(s, g - e) and (hf,s)(h - f, s) gives hf=eg.h - f = e - g. Writing δ=ge,\delta = g - e, we get E=(0,sδ2),E = \left(0, \frac{s - \delta}{2}\right), G=(s,s+δ2),G = \left(s, \frac{s + \delta}{2}\right), F=(2s5+δ2,0),F = \left(\frac{2s}{5} + \frac{\delta}{2}, 0\right), H=(2s5δ2,s),H = \left(\frac{2s}{5} - \frac{\delta}{2}, s\right), and EG=34EG = 34 gives s2+δ2=1156.s^2 + \delta^2 = 1156.

Intersecting lines EGEG and FHFH (and simplifying with s2+δ2=1156s^2 + \delta^2 = 1156) yields P=(s2s311560,  s2s2δ11560), \begin{aligned} P &= \\ &\left(\frac{s}{2} - \frac{s^3}{11560},\; \frac{s}{2} - \frac{s^2\delta}{11560}\right), \end{aligned} and the shoelace formula on A,E,P,HA, E, P, H then gives [AEPH]=s25s3δ231200.[AEPH] = \frac{s^2}{5} - \frac{s^3\delta}{231200}. Setting this equal to 2691360s2\frac{269}{1360}s^2 leaves 3s21360=s3δ231200,\frac{3s^2}{1360} = \frac{s^3\delta}{231200}, so sδ=510.s\delta = 510.

Now s2+δ2=1156s^2 + \delta^2 = 1156 and s2δ2=260100,s^2\delta^2 = 260100, so s2s^2 and δ2\delta^2 are the roots of t21156t+260100=0,t^2 - 1156t + 260100 = 0, which are 1156±5442=850\frac{1156 \pm 544}{2} = 850 and 306.306. Since δ=ge<s,|\delta| = |g - e| \lt s, the area is s2=850.s^2 = 850.

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