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Difference between revisions of "2006 AIME II Problems/Problem 13"

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== Solution ==
 
== Solution ==
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{{solution}}
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Let the first odd integer be <math>2n+1</math> with <math>n\geq 0</math>.
  
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The final odd integer is <math>2n+1 + 2(j-1) = 2(n+j) - 1</math>
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The odd integers form an arithmetic sequence with sum <math>j\left(\frac{(2n+1) + (2(n+j)-1)}{2}\right) = j(2n+j)</math>
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so <math>N = j(2n+j)</math>. <math>j</math> is a factor of <math>N</math>.
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Since <math>n\geq 0</math>, it follows that <math>2n+j \geq j</math> and <math>j\leq \sqrt{N}</math>
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Since there are exactly <math>5</math> values of <math>j</math> that satisfy the equation, there must be either <math>9</math> or <math>10</math> factors to <math>N</math>. This means <math>N=p_1^2p_2^2</math> or <math>N=p_1p_2^4</math>.
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Unfortunately, we cannot simply observe prime factorizations of <math>N</math> because the factor <math>(2n+j)</math> does not cover all integers for any given value of <math>j</math>
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Instead we do some casework:
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<b>If <math>N</math> is odd</b>, then <math>j</math> must also be odd. For every odd value of <math>j</math>, <math>2n+j</math> is also odd, making this case valid for all odd <math>j</math>. Looking at the forms above and the bound of 1000, N must be
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<math>(3^2\cdot5^2)</math>, <math>(3^2\cdot7^2)</math>, <math>(3^4\cdot5)</math>, <math>(3^4\cdot7)</math> or <math>(3^4\cdot 11)</math>
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Those give <math>5</math> possibilities for odd <math>N</math>
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<b>If <math>N</math> is even</b>, then <math>j</math> must also be even. Substituting <math>j=2k</math>, we get
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<math>N = 4k(n+k)</math>
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<math>\frac{N}{4} = k(n+k)</math>
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Now we can just look at all the prime factorizations since <math>(n+k)</math> cover the integers for any <math>k</math>. Note that our upper bound is now 250
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<math>\frac{N}{4} = (2^2\cdot3^2),(2^2\cdot5^2),(2^2\cdot7^2), (3^2\cdot5^2), (2^4\cdot3), (2^4\cdot5), (2^4\cdot7), (2^4\cdot11), (2^4\cdot13),</math> or <math>(3^4\cdot2)</math>
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Those give <math>10</math> possibilities for even <math>N</math>
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The total number of integers <math>N</math> is <math>5 + 10 = \boxed{015}</math>
 
== See also ==
 
== See also ==
 
{{AIME box|year=2006|n=II|num-b=12|num-a=14}}
 
{{AIME box|year=2006|n=II|num-b=12|num-a=14}}
  
 
[[Category:Intermediate Number Theory Problems]]
 
[[Category:Intermediate Number Theory Problems]]

Revision as of 22:52, 3 March 2008

Problem

How many integers $N$ less than 1000 can be written as the sum of $j$ consecutive positive odd integers from exactly 5 values of $j\ge 1$?

Solution

Let the first odd integer be $2n+1$ with $n\geq 0$.

The final odd integer is $2n+1 + 2(j-1) = 2(n+j) - 1$

The odd integers form an arithmetic sequence with sum $j\left(\frac{(2n+1) + (2(n+j)-1)}{2}\right) = j(2n+j)$

so $N = j(2n+j)$. $j$ is a factor of $N$.

Since $n\geq 0$, it follows that $2n+j \geq j$ and $j\leq \sqrt{N}$

Since there are exactly $5$ values of $j$ that satisfy the equation, there must be either $9$ or $10$ factors to $N$. This means $N=p_1^2p_2^2$ or $N=p_1p_2^4$. Unfortunately, we cannot simply observe prime factorizations of $N$ because the factor $(2n+j)$ does not cover all integers for any given value of $j$


Instead we do some casework:

If $N$ is odd, then $j$ must also be odd. For every odd value of $j$, $2n+j$ is also odd, making this case valid for all odd $j$. Looking at the forms above and the bound of 1000, N must be

$(3^2\cdot5^2)$, $(3^2\cdot7^2)$, $(3^4\cdot5)$, $(3^4\cdot7)$ or $(3^4\cdot 11)$

Those give $5$ possibilities for odd $N$


If $N$ is even, then $j$ must also be even. Substituting $j=2k$, we get

$N = 4k(n+k)$

$\frac{N}{4} = k(n+k)$

Now we can just look at all the prime factorizations since $(n+k)$ cover the integers for any $k$. Note that our upper bound is now 250

$\frac{N}{4} = (2^2\cdot3^2),(2^2\cdot5^2),(2^2\cdot7^2), (3^2\cdot5^2), (2^4\cdot3), (2^4\cdot5), (2^4\cdot7), (2^4\cdot11), (2^4\cdot13),$ or $(3^4\cdot2)$

Those give $10$ possibilities for even $N$

The total number of integers $N$ is $5 + 10 = \boxed{015}$

See also

2006 AIME II (Problems • Answer Key • Resources)
Preceded by
Problem 12 		Followed by
Problem 14
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15
All AIME Problems and Solutions
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