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Difference between revisions of "2013 AMC 10B Problems/Problem 24"

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==Solution==
 
==Solution==
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A positive integer with only four positive divisors has its prime factorization in the form of <math>a*b</math>, where <math>a</math> and <math>b</math> are both prime positive integers or <math>c^3</math> where <math>c</math> is a prime. One can easily deduce that none of the numbers are even near a cube so that case is finished. We now look at the case of <math>a*b</math>. The four factors of this number would be <math>1</math>, <math>a</math>, <math>b</math>, and <math>ab</math>.  The sum of these would be <math>ab+a+b+1</math>, which can be factored into the form <math>(a+1)(b+1)</math>. Easily we can see that Now we can take cases again.  
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A positive integer with only four positive divisors has its prime factorization in the form of <math>a*b</math>, where <math>a</math> and <math>b</math> are both prime positive integers or <math>c^3</math> where <math>c</math> is a prime. One can easily deduce that none of the numbers are even near a cube so that case is finished. We now look at the case of <math>a*b</math>. The four factors of this number would be <math>1</math>, <math>a</math>, <math>b</math>, and <math>ab</math>.  The sum of these would be <math>ab+a+b+1</math>, which can be factored into the form <math>(a+1)(b+1)</math>. Easily we can see that now we can take cases again.  
  
 
Case 1: Either <math>a</math> or <math>b</math> is 2.  
 
Case 1: Either <math>a</math> or <math>b</math> is 2.  
  
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If this is true then we have to have that one of <math>(a+1)</math> or <math>(b+1)</math> is odd and that one is 3. The other is still even.  So we have that in this case the only numbers that work are even multiples of 3 which are 2010 and 2016. So we just have to check if either <math>\frac{2016}{3} - 1</math>  or <math>\frac{2010}{3} - 1</math> is a prime. We see that <math>2016</math> is the only one that works in this case.  
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If this is true then we have to have that one of <math>(a+1)</math> or <math>(b+1)</math> is odd and that one is 3. The other is still even.  So we have that in this case the only numbers that work are even multiples of 3 which are 2010 and 2016. So we just have to check if either <math>\frac{2016}{3} - 1</math>  or <math>\frac{2010}{3} - 1</math> is a prime. We see that in this case none of them work.
  
 
Case 2: Both <math>a</math> and <math>b</math> are odd primes.  
 
Case 2: Both <math>a</math> and <math>b</math> are odd primes.  
  
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This implies that both <math>(a+1)</math> and <math>(b+1)</math> are even which implies that in this case the number must be divisible by <math>4</math>. This leaves only <math>2012</math> and <math>2016</math>. We know <math>2016</math> works so it suffices to check whether <math>2012</math> works.
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This implies that both <math>(a+1)</math> and <math>(b+1)</math> are even which implies that in this case the number must be divisible by <math>4</math>. This leaves only <math>2012</math> and <math>2016</math>.
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<math>2012=4*503</math> so we have that a factor of <math>2</math> must go to both <math>(a+1)</math> and <math>(b+1)</math>. So we have that <math>(a+1)</math> and <math>(b+1)</math> equal the numbers <math>(2+503)(2+1)</math>, but this contradicts our assumption for the case.  Thus the answer is <math>\boxed{\textbf{(A)}\ 1}</math> as <math>2016</math> is the only solution.
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<math>2012=4*503</math> so we have that a factor of <math>2</math> must go to both <math>(a+1)</math> and <math>(b+1)</math>. So we have that <math>(a+1)</math> and <math>(b+1)</math> equal the numbers <math>(502 + 1)(3 + 1)</math>, but 502 is not an odd prime, so 2012 does not work.  <math>2016 = 4 * 504</math> so we have <math>(503 + 1)(3 + 1). 503 and 3 are both odd primes, so 2016 is a solution. Thus the answer is </math>\boxed{\textbf{(A)}\ 1}$.
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**Note: <math>\frac{2016}{3} - 1</math> = <math>672</math> - <math>1</math> = <math>671</math> = <math>61</math> * <math>11</math> , so why did we get the right answer?**
 
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**Note: In Case 1, isn't one of <math>(a+1)</math> or <math>(b+1)</math> even and one is 3, as one of a and b is 2 and one is an odd prime?**
 
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**Note: 2016 actually does not work in the first case, but it does work in the second case, so it is indeed the only solution.**
 
  
 
== See also ==
 
== See also ==

Revision as of 13:41, 23 December 2014

Problem

A positive integer $n$ is nice if there is a positive integer $m$ with exactly four positive divisors (including $1$ and $m$) such that the sum of the four divisors is equal to $n$. How many numbers in the set $\{ 2010,2011,2012,\dotsc,2019 \}$ are nice?


$\textbf{(A)}\ 1 \qquad\textbf{(B)}\ 2 \qquad\textbf{(C)}\ 3 \qquad\textbf{(D)}\ 4 \qquad\textbf{(E)}\ 5$

Solution

A positive integer with only four positive divisors has its prime factorization in the form of $a*b$, where $a$ and $b$ are both prime positive integers or $c^3$ where $c$ is a prime. One can easily deduce that none of the numbers are even near a cube so that case is finished. We now look at the case of $a*b$. The four factors of this number would be $1$, $a$, $b$, and $ab$. The sum of these would be $ab+a+b+1$, which can be factored into the form $(a+1)(b+1)$. Easily we can see that now we can take cases again.

Case 1: Either $a$ or $b$ is 2.

If this is true then we have to have that one of $(a+1)$ or $(b+1)$ is odd and that one is 3. The other is still even. So we have that in this case the only numbers that work are even multiples of 3 which are 2010 and 2016. So we just have to check if either $\frac{2016}{3} - 1$ or $\frac{2010}{3} - 1$ is a prime. We see that in this case none of them work.

Case 2: Both $a$ and $b$ are odd primes.

This implies that both $(a+1)$ and $(b+1)$ are even which implies that in this case the number must be divisible by $4$. This leaves only $2012$ and $2016$. $2012=4*503$ so we have that a factor of $2$ must go to both $(a+1)$ and $(b+1)$. So we have that $(a+1)$ and $(b+1)$ equal the numbers $(502 + 1)(3 + 1)$, but 502 is not an odd prime, so 2012 does not work. $2016 = 4 * 504$ so we have $(503 + 1)(3 + 1). 503 and 3 are both odd primes, so 2016 is a solution. Thus the answer is$\boxed{\textbf{(A)}\ 1}$.

See also

2013 AMC 10B (Problems • Answer Key • Resources)
Preceded by
Problem 23 		Followed by
Problem 25
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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