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1990 AIME Problems/Problem 3

Revision as of 14:20, 29 August 2020 by Jackshi2006 (talk | contribs) (→‎Solution 2)

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.


$s * 116 = 118 * 116 - 116$


$s = 118 - 1$


$s = \boxed{117}$


-jackshi2006

See also

1990 AIME (Problems • Answer Key • Resources)
Preceded by
Problem 2 		Followed by
Problem 4
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15
All AIME Problems and Solutions

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