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Difference between revisions of "2012 AMC 10B Problems/Problem 23"

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We can write the volume of a tetrahedron as Bh/3 and revise the formula for the volume of a cube to be Bh. Also note that the height of the tetrahedron goes through the space diagonal of the cube. So, the remaining part of the cube's space diagonal should be 2/3 of the original space diagonal. Hence, the length is (2rt3)/2.
 
We can write the volume of a tetrahedron as Bh/3 and revise the formula for the volume of a cube to be Bh. Also note that the height of the tetrahedron goes through the space diagonal of the cube. So, the remaining part of the cube's space diagonal should be 2/3 of the original space diagonal. Hence, the length is (2rt3)/2.
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==Solution 2==
 
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Note that the volume of a tetrahedron is <math>\frac{Bh}/{3}</math> and the area of a cube can be revised as being <math>Bh</math>. Also note that the height of the tetrahedron goes through the space diagonal of the cube. Knowing this, we can find the remaining part of the space diagonal. <math>\sqrt{3}-\frac{\sqrt{3}{3}=\frac{2{\sqrt{3}}{3}</math>. (The whole diagonal minus the the height of the tetrahedron)
 

Revision as of 14:10, 8 January 2017

Problem

A solid tetrahedron is sliced off a solid wooden unit cube by a plane passing through two nonadjacent vertices on one face and one vertex on the opposite face not adjacent to either of the first two vertices. The tetrahedron is discarded and the remaining portion of the cube is placed on a table with the cut surface face down. What is the height of this object?

$\textbf{(A)}\ \frac{\sqrt{3}}{3} \qquad\textbf{(B)}\ \frac{2 \sqrt{2}}{3}\qquad\textbf{(C)}\ 1\qquad\textbf{(D)}\ \frac{2 \sqrt{3}}{3}\qquad\textbf{(E)}\ \sqrt{2}$

Solution

This tetrahedron has the 4 vertices in these positions: on a corner (lets call this $A$) of the cube, and the other three corners ($B$, $C$, and $D$) adjacent to this corner. We can find the height of the remaining portion of the cube by finding the height of the tetrahedron. We can find the height of this tetrahedron in perspective to the equilateral triangle base (we call this height $x$) by finding the area of the tetrahedron in two ways. $\frac{1 \times 1}{2}$ is the area of the isosceles base of the tetrahedron. Multiply by the height, $1$, and divide by $3$, we have the volume of the tetrahedron as $\frac{1}{6}$. We set this area equal to one-third the product of our desired height and the area of the equilateral triangle base. First, find the area of the equilateral triangle: $[BCD]=\frac{\sqrt{2}^2 \times \sqrt{3}}{4}=\frac{\sqrt{3}}{2}$. So we have: $\frac{1}{3} \cdot \frac{\sqrt{3}}{2} \cdot x=\frac{1}{6}$, and so $x=\frac{\sqrt{3}}{3}$.

Since we know what the height is, we can find the height of the remaining structure. The height of the structure if the tetrahedron was still on would simply be the space diagonal of the cube, $\sqrt{3}$, so we just subtract $\frac{\sqrt{3}}{3}$ from $\sqrt{3}$ to get $\frac{2\sqrt{3}}{3}$, or $\boxed{\textbf{(D)}}.$

Solution 2

We can write the volume of a tetrahedron as Bh/3 and revise the formula for the volume of a cube to be Bh. Also note that the height of the tetrahedron goes through the space diagonal of the cube. So, the remaining part of the cube's space diagonal should be 2/3 of the original space diagonal. Hence, the length is (2rt3)/2.

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