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Difference between revisions of "2002 AMC 12B Problems/Problem 13"

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== Solution ==
 
== Solution ==
  
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=== Solution 1 ===
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== Solution 1 ==
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Let the first term be <math>x</math> and the common difference be <math>d.</math>
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Thus, we know the sum of terms is <math>9(2x+17d).</math>
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Because <math>9</math> is already a perfect square, we know that <math>2x+17d</math> must also be a perfect square.
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Since we want the smallest value of the sum, we know that <math>25=2x+17 \rightarrow x=4</math> and <math>d=1.</math>
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Thus, the answer is <math>9*25 = \boxed{225}.</math>
  
 
== Solution 2 ==
 
== Solution 2 ==

Revision as of 20:20, 31 August 2020

Problem

The sum of $18$ consecutive positive integers is a perfect square. The smallest possible value of this sum is

$\mathrm{(A)}\ 169 \qquad\mathrm{(B)}\ 225 \qquad\mathrm{(C)}\ 289 \qquad\mathrm{(D)}\ 361 \qquad\mathrm{(E)}\ 441$

Solution

Solution 1

Let the first term be $x$ and the common difference be $d.$

Thus, we know the sum of terms is $9(2x+17d).$

Because $9$ is already a perfect square, we know that $2x+17d$ must also be a perfect square.

Since we want the smallest value of the sum, we know that $25=2x+17 \rightarrow x=4$ and $d=1.$

Thus, the answer is $9*25 = \boxed{225}.$

Solution 2

Let $a, a+1, \ldots, a + 17$ be the consecutive positive integers. Their sum, $18a + \frac{17(18)}{2} = 9(2a+17)$, is a perfect square. Since $9$ is a perfect square, it follows that $2a + 17$ is a perfect square. The smallest possible such perfect square is $25$ when $a = 4$, and the sum is $225 \Rightarrow \mathrm{(B)}$.

Solution 2

Notice that all five choices given are perfect squares.

Let $a$ be the smallest number, we have \[a+(a+1)+(a+2)+...+(a+17)=18a+\sum_{k=1}^{17}k=18a+153\]

Subtract $153$ from each of the choices and then check its divisibility by $18$, we have $225$ as the smallest possible sum. $\mathrm {(B)}$

~ Nafer

See also

2002 AMC 12B (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 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 12 Problems and Solutions

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