Difference between revisions of "1961 AHSME Problems/Problem 39"

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== Problem 39==
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== Problem ==
  
 
Any five points are taken inside or on a square with side length <math>1</math>. Let a be the smallest possible number with the  
 
Any five points are taken inside or on a square with side length <math>1</math>. Let a be the smallest possible number with the  
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Hence our answer is <math>\fbox{B}</math>.
 
Hence our answer is <math>\fbox{B}</math>.
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==See Also==
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{{AHSME 40p box|year=1961|num-b=38|after=num-a=40}}
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[[Category:Intermediate Geometry Problems]]
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{{MAA Notice}}

Revision as of 13:55, 15 July 2018

Problem

Any five points are taken inside or on a square with side length $1$. Let a be the smallest possible number with the property that it is always possible to select one pair of points from these five such that the distance between them is equal to or less than $a$. Then $a$ is:

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

Solution

Partition the unit square into four smaller squares of sidelength $\frac{1}{2}$. Each of the five points lies in one of these squares, and so by the Pigeonhole Principle, there exists two points in the same $\frac{1}{2}\times \frac{1}{2}$ square - the maximum possible distance between them being $\frac{\sqrt{2}}{2}$ by Pythagoras.

Hence our answer is $\fbox{B}$.

See Also

1961 AHSC (ProblemsAnswer KeyResources)
Preceded by
Problem 38
Followed by
num-a=40
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 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
All AHSME Problems and Solutions

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