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Difference between revisions of "1990 AIME Problems/Problem 3"

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Let <math>P_1^{}</math> be a [[Regular polygon|regular]] <math>r~\mbox{gon}</math> and <math>P_2^{}</math> be a regular <math>s~\mbox{gon}</math> <math>(r\geq s\geq 3)</math> such that each [[interior angle]] of <math>P_1^{}</math> is <math>\frac{59}{58}</math> as large as each interior angle of <math>P_2^{}</math>. What's the largest possible value of <math>s_{}^{}</math>?
 
Let <math>P_1^{}</math> be a [[Regular polygon|regular]] <math>r~\mbox{gon}</math> and <math>P_2^{}</math> be a regular <math>s~\mbox{gon}</math> <math>(r\geq s\geq 3)</math> such that each [[interior angle]] of <math>P_1^{}</math> is <math>\frac{59}{58}</math> as large as each interior angle of <math>P_2^{}</math>. What's the largest possible value of <math>s_{}^{}</math>?
  
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== Solution ==
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== Solution 1==
 
The formula for the interior angle of a regular sided [[polygon]] is <math>\frac{(n-2)180}{n}</math>.  
 
The formula for the interior angle of a regular sided [[polygon]] is <math>\frac{(n-2)180}{n}</math>.  
  
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This is achievable because the denominator is <math>1</math>, making <math>r</math> a positive number <math>116 \cdot 117</math> and <math>s = \boxed{117}</math>.
 
This is achievable because the denominator is <math>1</math>, making <math>r</math> a positive number <math>116 \cdot 117</math> and <math>s = \boxed{117}</math>.
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== Solution 2==
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Like above, use the formula for the interior angles of a regular sided [[polygon]].
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<math>\frac{(r-2)180}{r} = \frac{59}{58} * \frac{(s-2)180}{s}</math>
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<math>59 * 180 * (s-2) * r = 58 * 180 * (r-2) * s</math>
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<math>59 * (rs - 2r) = 58 * (rs - 2s)</math>
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<math>rs - 118r = -116s</math>
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<math>rs = 118r-116s</math>
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This equation tells us <math>s</math> divides <math>118r</math>. If <math>s</math> specifically divides 118 then the highest it can be is 118. However, this gives an equation with no solution. The second largest possibility in this case is <math>s=59</math>, which does give a solution: <math>s=59, r=116</math>. Although, the problem asks for <math>s</math>, not <math>r</math>. The only conceivable reasoning behind this is that <math>r</math> is greater than 1000. This prompts us to look into the second case, where <math>s</math> divides <math>r</math>. Make <math>r = s * k</math>. Rewrite the equation using this new information.
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<math>s * s * k = 118 * s * k - 116 * s</math>
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<math>s * k = 118 * k - 116</math>
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Now we now k divides 116. The larger k is, the larger s will be, so we set k to be the maximum, 116.
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<math>s * 116 = 118 * 116 - 116</math>
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<math>s = 118 - 1</math>
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<math>s = \boxed{117}</math>
  
 
== See also ==
 
== See also ==

Revision as of 14:20, 29 August 2020

Problem

Let $P_1^{}$ be a regular $r~\mbox{gon}$ and $P_2^{}$ be a regular $s~\mbox{gon}$ $(r\geq s\geq 3)$ such that each interior angle of $P_1^{}$ is $\frac{59}{58}$ as large as each interior angle of $P_2^{}$. What's the largest possible value of $s_{}^{}$?

Solution 1

The formula for the interior angle of a regular sided polygon is $\frac{(n-2)180}{n}$.

Thus, $\frac{\frac{(r-2)180}{r}}{\frac{(s-2)180}{s}} = \frac{59}{58}$. Cross multiplying and simplifying, we get $\frac{58(r-2)}{r} = \frac{59(s-2)}{s}$. Cross multiply and combine like terms again to yield $58rs - 58 \cdot 2s = 59rs - 59 \cdot 2r \Longrightarrow 118r - 116s = rs$. Solving for $r$, we get $r = \frac{116s}{118 - s}$.

$r \ge 0$ and $s \ge 0$, making the numerator of the fraction positive. To make the denominator positive, $s < 118$; the largest possible value of $s$ is $117$.

This is achievable because the denominator is $1$, making $r$ a positive number $116 \cdot 117$ and $s = \boxed{117}$.

Solution 2

Like above, use the formula for the interior angles of a regular sided polygon.


$\frac{(r-2)180}{r} = \frac{59}{58} * \frac{(s-2)180}{s}$


$59 * 180 * (s-2) * r = 58 * 180 * (r-2) * s$


$59 * (rs - 2r) = 58 * (rs - 2s)$


$rs - 118r = -116s$


$rs = 118r-116s$


This equation tells us $s$ divides $118r$. If $s$ specifically divides 118 then the highest it can be is 118. However, this gives an equation with no solution. The second largest possibility in this case is $s=59$, which does give a solution: $s=59, r=116$. Although, the problem asks for $s$, not $r$. The only conceivable reasoning behind this is that $r$ is greater than 1000. This prompts us to look into the second case, where $s$ divides $r$. Make $r = s * k$. Rewrite the equation using this new information.


$s * s * k = 118 * s * k - 116 * s$


$s * k = 118 * k - 116$


Now we now k divides 116. The larger k is, the larger s will be, so we set k to be the maximum, 116.


$s * 116 = 118 * 116 - 116$


$s = 118 - 1$


$s = \boxed{117}$

See also

1990 AIME (Problems • Answer Key • Resources)
Preceded by
Problem 2 		Followed by
Problem 4
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All AIME Problems and Solutions

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