2019 AIME II 第 7 题

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7.

三角形 ABCABC 的边长为 AB=120AB = 120BC=220BC = 220AC=180AC = 180 作直线 A\ell_AB\ell_BC\ell_C,分别平行于 BC\overline{BC}AC\overline{AC}AB\overline{AB},使得 A\ell_AB\ell_BC\ell_CABC\triangle ABC 内部的交线段长度分别为 555545451515。求以直线 A\ell_AB\ell_BC\ell_C 为边所在直线的三角形的周长。

Triangle ABCABC has side lengths AB=120,AB = 120, BC=220,BC = 220, and AC=180.AC = 180. Lines A,\ell_A, B,\ell_B, and C\ell_C are drawn parallel to BC,\overline{BC}, AC,\overline{AC}, and AB,\overline{AB}, respectively, such that the intersections of A,\ell_A, B,\ell_B, and C\ell_C with the interior of ABC\triangle ABC are segments of lengths 55,55, 45,45, and 15,15, respectively. Find the perimeter of the triangle whose sides lie on lines A,\ell_A, B,\ell_B, and C.\ell_C.

答案:715
知识点:相似平行线
难度评级:2790
解答:

对点 PP,令 α\alphaPP 到直线 BCBC 的距离除以从 AA, 引出的高,类似地定义 β\beta(到 CACA)和 γ\gamma(到 ABAB);对内部点有 α+β+γ=1\alpha + \beta + \gamma = 1,因为 [PBC]+[PCA][PBC] + [PCA] +[PAB]=[ABC]+ [PAB] = [ABC]。一条平行于 BC\overline{BC}、位于层级 α\alpha 的弦,会在 AA 处截出与 ABCABC 相似、比例为 1α1 - \alpha 的三角形,所以它的长度为 220(1α)220(1 - \alpha) 长度 5555 的弦给出 1α=141 - \alpha = \frac{1}{4},所以 A\ell_A 是直线 α=34\alpha = \frac{3}{4};类似地 45=180(1β)45 = 180(1 - \beta) 使 B\ell_B 位于 β=34\beta = \frac{3}{4},而 15=120(1γ)15 = 120(1 - \gamma) 使 C\ell_C 位于 γ=78\gamma = \frac{7}{8}

沿任意平行于 BC\overline{BC} 的直线,坐标 β\beta 线性变化;在三角形内部层级 α\alpha 的弦上,β\beta 经过长度为 1α1 - \alpha 的区间,而该弦长为 220(1α)220(1 - \alpha);因此,一段平行于 BC\overline{BC}、端点坐标差为 Δβ\Delta\beta 的线段长度为 220Δβ220\,|\Delta\beta|。新三角形在 A\ell_A 上的边从 AB\ell_A \cap \ell_B(此时 β=34\beta = \frac{3}{4})到 AC\ell_A \cap \ell_C(此时 β=13478=58\beta = 1 - \frac{3}{4} - \frac{7}{8} = -\frac{5}{8})。其长度为 220(34+58)=220118.220\left(\frac{3}{4} + \frac{5}{8}\right) = 220 \cdot \frac{11}{8}.

因为这三条直线分别平行于 ABCABC 的三边,它们围成的三角形与 ABCABC, 相似,此处相似比为 118\frac{11}{8}。其周长为 118(120+220+180)\frac{11}{8}(120 + 220 + 180) =118520=715= \frac{11}{8} \cdot 520 = 715

For a point P,P, let α\alpha be the distance from PP to line BCBC divided by the length of the altitude from A,A, and define β\beta (to CACA) and γ\gamma (to ABAB) similarly; then α+β+γ=1\alpha + \beta + \gamma = 1 for points inside, since [PBC]+[PCA][PBC] + [PCA] +[PAB]=[ABC].+ [PAB] = [ABC]. A chord parallel to BC\overline{BC} at level α\alpha cuts off a triangle at AA similar to ABCABC with ratio 1α,1 - \alpha, so its length is 220(1α).220(1 - \alpha). The chord of length 5555 gives 1α=14,1 - \alpha = \frac{1}{4}, so A\ell_A is the line α=34;\alpha = \frac{3}{4}; similarly 45=180(1β)45 = 180(1 - \beta) puts B\ell_B at β=34,\beta = \frac{3}{4}, and 15=120(1γ)15 = 120(1 - \gamma) puts C\ell_C at γ=78.\gamma = \frac{7}{8}.

Along any line parallel to BC,\overline{BC}, the coordinate β\beta varies linearly, and on the chord at level α\alpha inside the triangle, β\beta runs over an interval of length 1α1 - \alpha while the chord has length 220(1α);220(1 - \alpha); hence a segment parallel to BC\overline{BC} with endpoints differing by Δβ\Delta\beta has length 220Δβ.220\,|\Delta\beta|. The side of the new triangle on A\ell_A runs from AB,\ell_A \cap \ell_B, where β=34,\beta = \frac{3}{4}, to AC,\ell_A \cap \ell_C, where β=13478=58.\beta = 1 - \frac{3}{4} - \frac{7}{8} = -\frac{5}{8}. Its length is 220(34+58)=220118.220\left(\frac{3}{4} + \frac{5}{8}\right) = 220 \cdot \frac{11}{8}.

Since the three lines are parallel to the sides of ABC,ABC, the triangle they bound is similar to ABC,ABC, here with ratio 118.\frac{11}{8}. Its perimeter is 118(120+220+180)\frac{11}{8}(120 + 220 + 180) =118520=715.= \frac{11}{8} \cdot 520 = 715.

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