2020 AMC 10B Problems/Problem 20

Problem

Let $B$ be a right rectangular prism (box) with edges lengths $1,$ $3,$ and $4$ , together with its interior. For real $r\geq0$ , let $S(r)$ be the set of points in $3$ -dimensional space that lie within a distance $r$ of some point $B$ . The volume of $S(r)$ can be expressed as $ar^{3} + br^{2} + cr +d$ , where $a,$ $b,$ $c,$ and $d$ are positive real numbers. What is $\frac{bc}{ad}?$

$\textbf{(A) } 6 \qquad\textbf{(B) } 19 \qquad\textbf{(C) } 24 \qquad\textbf{(D) } 26 \qquad\textbf{(E) } 38$

Solution

Split the volume into 4 regions:

1. The rectangular prism itself

2. The extensions of the faces of B

3. The quarter cylinders at each edge of B

4. The one-eighth spheres at each corner of B.

Region 1: The volume of B is 12, so $d=12$

Region 2: The volume is equal to the surface area of B times r. The surface area can easily be computed to be 38, so $c=38$ .

Region 3: The volume of each quarter cylinder is equal to $(\pi*r^2*h)/4$ . The sum of all such cylinders must equal $(\pi*r^2)/4$ times the sum of the edge lengths. This can easily be computed as 32, so the sum of the volumes of the quarter cylinders is $8\pi*r^2$ . $b=8\pi$

Region 4: There is an eighth of a sphere of radius r at each corner. Since there are 8 corners, these add up to one full sphere of radius r. The volume of this sphere is $\frac{4}{3}\pi*r^3$ . $a=\frac{4}{3}$ .

Using these values, $\frac{(8\pi)(38)}{(4\pi/3)(12)} = 19$ (D)

~DrJoyo

Video Solution

https://youtu.be/3BvJeZU3T-M

~IceMatrix

2020 AMC 10B (Problems • Answer Key • Resources)
Preceded by Problem 19	Followed by Problem 21
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
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2020 AMC 10B Problems/Problem 20

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Video Solution

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