1986 AIME 第 14 题

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14.

长方体 PP 的一条体对角线与各条不相交的棱之间的最短距离分别为 252\sqrt53013\frac{30}{\sqrt{13}}1510\frac{15}{\sqrt{10}}。求 PP 的体积。

The shortest distances between an interior diagonal of a rectangular parallelepiped PP and the edges it does not meet are 25,2\sqrt5, 3013,\frac{30}{\sqrt{13}}, and 1510.\frac{15}{\sqrt{10}}. Determine the volume of P.P.

答案:750
知识点:长方体方程组向量
难度评级:3060
小提示:

设三条棱长为 aabbcc,并使用异面直线间距离的向量公式

Let the side lengths be a,a, b,b, and cc and use a vector formula for the distance between skew lines

大提示:

对各距离的平方取倒数,可使方程关于 1a2\frac{1}{a^2}1b2\frac{1}{b^2}1c2\frac{1}{c^2} 成为线性方程

Taking reciprocals of the squared distances makes the equations linear in 1a2,\frac{1}{a^2}, 1b2,\frac{1}{b^2}, and 1c2\frac{1}{c^2}

解答:

设三条棱长为 aabbcc。例如,方向为 (a,b,c)(a,b,c) 的体对角线到一条与其不相交且平行于 aa 方向的棱的距离为 bcb2+c2 \frac{bc}{\sqrt{b^2+c^2}}\text{。}另外两个距离可由循环置换得到。我们可以依题中次序将三个已知距离分别对应到这三个方向,因为改变对应关系只会置换三条棱长。

X=1a2X=\frac{1}{a^2}Y=1b2Y=\frac{1}{b^2},且 Z=1c2Z=\frac{1}{c^2}。对各距离的平方取倒数,得到 Y+Z=120,X+Z=13900,X+Y=245 \begin{aligned} Y+Z&=\frac1{20},\\ X+Z&=\frac{13}{900},\\ X+Y&=\frac2{45} \end{aligned}\text{。}解得 X=1225,Y=125,Z=1100 \begin{aligned} X&=\frac1{225},\\ Y&=\frac1{25},\\ Z&=\frac1{100} \end{aligned}\text{。}因此三条棱长为 1515551010,体积为 15510=75015\cdot5\cdot10=750

Let the side lengths be a,a, b,b, and c.c. For example, the distance from the space diagonal with direction (a,b,c)(a,b,c) to a nonintersecting edge parallel to the aa-direction is bcb2+c2. \frac{bc}{\sqrt{b^2+c^2}}. The other two distances are obtained cyclically. We may assign the three given distances to these three directions in the listed order, since permuting them only permutes the side lengths.

Put X=1a2,X=\frac{1}{a^2}, Y=1b2,Y=\frac{1}{b^2}, and Z=1c2.Z=\frac{1}{c^2}. Taking reciprocal squares gives Y+Z=120,X+Z=13900,X+Y=245. \begin{aligned} Y+Z&=\frac1{20},\\ X+Z&=\frac{13}{900},\\ X+Y&=\frac2{45}. \end{aligned} Solving, X=1225,Y=125,Z=1100. \begin{aligned} X&=\frac1{225},\\ Y&=\frac1{25},\\ Z&=\frac1{100}. \end{aligned} Hence the side lengths are 15,15, 5,5, 10,10, and the volume is 15510=750.15\cdot5\cdot10=750.

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