2020 AMC 10B Problems/Problem 20

Revision as of 03:54, 8 February 2020 by Drjoyo (talk | contribs) (Solution)

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

Video Solution

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

~IceMatrix

See Also

2020 AMC 10B (ProblemsAnswer KeyResources)
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
All AMC 10 Problems and Solutions

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