1999 AIME 第 15 题

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15.

考虑一个纸三角形,其顶点为 (0,0)(0, 0)(34,0)(34, 0)(16,24)(16, 24)。它的中点三角形的顶点是原三角形三边的中点。沿着中点三角形的三条边折叠这个三角形,形成一个三棱锥。这个三棱锥的体积是多少?

Consider the paper triangle whose vertices are (0,0),(0, 0), (34,0),(34, 0), and (16,24).(16, 24). The vertices of its midpoint triangle are the midpoints of its sides. A triangular pyramid is formed by folding the triangle along the sides of its midpoint triangle. What is the volume of this pyramid?

答案:408
知识点:折纸立体几何坐标几何体积
难度评级:2990
解答:

中点为 M1=(17,0)M_1 = (17, 0)M2=(25,12)M_2 = (25, 12), 和 M3=(8,12)M_3 = (8, 12)。 沿中点三角形三边把三个角折起后, 三个原顶点会在同一个顶点 QQ 处相遇(每对被粘合的半边长度相等)。顶点保持折叠后的距离: QM1=17QM_1 = 17M1M_1 所平分的长为 3434 的边的一半), QM2=15QM_2 = 153030 的一半),且 QM3=413QM_3 = 4\sqrt{13}162+242=813\sqrt{16^2 + 24^2} = 8\sqrt{13} 的一半)。

把中点三角形保持在平面 z=0z = 0 中,设 Q=(x,y,z)Q = (x, y, z)。 用 QM22=225|Q - M_2|^2 = 225 减去 QM32=208|Q - M_3|^2 = 208(x25)2(x8)2=17(x - 25)^2 - (x - 8)^2 = 17, 所以 x=16x = 16; 用 QM12=289|Q - M_1|^2 = 289 减去 QM22=225|Q - M_2|^2 = 2252x+3y=682x + 3y = 68, 所以 y=12y = 12。 于是 z2z^2 =289(1617)2= 289 - (16 - 17)^2 122- 12^2 =144= 144, 顶点高度为 z=12z = 12

底面是中点三角形,面积是原三角形面积 123424=408\frac{1}{2} \cdot 34 \cdot 24 = 408 的四分之一,即 102102。 体积为 1310212=408.\frac{1}{3} \cdot 102 \cdot 12 = 408.

The midpoints are M1=(17,0),M_1 = (17, 0), M2=(25,12),M_2 = (25, 12), and M3=(8,12).M_3 = (8, 12). Folding the three corner triangles up along the sides of the midpoint triangle brings the corners together at one apex QQ (each pair of glued half-sides has equal length). The apex keeps its folded distances: QM1=17QM_1 = 17 (half of the side of length 3434 that M1M_1 bisects), QM2=15QM_2 = 15 (half of 3030), and QM3=413QM_3 = 4\sqrt{13} (half of 162+242=813\sqrt{16^2 + 24^2} = 8\sqrt{13}).

Keep the midpoint triangle in the plane z=0z = 0 and let Q=(x,y,z).Q = (x, y, z). Subtracting QM32=208|Q - M_3|^2 = 208 from QM22=225|Q - M_2|^2 = 225 gives (x25)2(x8)2=17,(x - 25)^2 - (x - 8)^2 = 17, so x=16;x = 16; subtracting QM22=225|Q - M_2|^2 = 225 from QM12=289|Q - M_1|^2 = 289 gives 2x+3y=68,2x + 3y = 68, so y=12.y = 12. Then z2z^2 =289(1617)2= 289 - (16 - 17)^2 122- 12^2 =144,= 144, so the apex is at height z=12.z = 12.

The base is the midpoint triangle, with area one quarter of the original triangle's 123424=408,\frac{1}{2} \cdot 34 \cdot 24 = 408, i.e. 102.102. The volume is 1310212=408.\frac{1}{3} \cdot 102 \cdot 12 = 408.

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