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Difference between revisions of "2015 AMC 12B Problems/Problem 24"

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On the other hand, if they lie on opposite sides, then it is clear that there is only one possibility, and then it is clear that <math>O_1 R + O_2 R = O_1 O_2 = 39</math>. Therefore, we obtain exactly four possible centers, and the sum of the desired lengths is <math>57+96+39 = \boxed{\textbf{(D)}\; 192}</math>.
 
On the other hand, if they lie on opposite sides, then it is clear that there is only one possibility, and then it is clear that <math>O_1 R + O_2 R = O_1 O_2 = 39</math>. Therefore, we obtain exactly four possible centers, and the sum of the desired lengths is <math>57+96+39 = \boxed{\textbf{(D)}\; 192}</math>.
  
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==Number-Intensive Solution==
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Start by drawing <math>PQ</math> first, because trying to get all four circles down will take you a few years. Next, because all circles have <math>P</math> and <math>Q</math> on them, and since all points on a circle are equidistant from the center, all circle centers lie on the perpendicular bisector of <math>PQ</math>, and point <math>R</math> is on this bisector.
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In order for all the circle radii to be different (because the circles can't be congruent), two circle centers are on the same side of <math>PQ</math>, and two are straddling it. For the latter two circles- just call them <math>A</math> and <math>B</math>- clearly <math>AR+BR</math> is 39.
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Now, let's take the next case. Then <math>C</math> and <math>D</math> lie on the same side. Construct the triangles from your picture, and use the Pythagorean Theorem (maybe divide all lengths by 3 to lessen big numbers) and then you get that the distance from <math>R</math> to the closest circle center is <math>57</math>. Therefore, the answer is <math>39+2*57+39=192 \boxed{(D)}</math>.
 
==See Also==
 
==See Also==
 
{{AMC12 box|year=2015|ab=B|num-a=25|num-b=23}}
 
{{AMC12 box|year=2015|ab=B|num-a=25|num-b=23}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Revision as of 11:56, 4 February 2018

Problem

Four circles, no two of which are congruent, have centers at $A$, $B$, $C$, and $D$, and points $P$ and $Q$ lie on all four circles. The radius of circle $A$ is $\tfrac{5}{8}$ times the radius of circle $B$, and the radius of circle $C$ is $\tfrac{5}{8}$ times the radius of circle $D$. Furthermore, $AB = CD = 39$ and $PQ = 48$. Let $R$ be the midpoint of $\overline{PQ}$. What is $AR+BR+CR+DR$ ?

$\textbf{(A)}\; 180 \qquad\textbf{(B)}\; 184 \qquad\textbf{(C)}\; 188 \qquad\textbf{(D)}\; 192\qquad\textbf{(E)}\; 196$

Solution

First, note that $PQ$ lies on the radical axis of any of the pairs of circles. Suppose that $O_1$ and $O_2$ are the centers of two circles $C_1$ and $C_2$ that intersect exactly at $P$ and $Q$, with $O_1$ and $O_2$ lying on the same side of $PQ$, and $O_1 O_2=39$. Let $x=O_1 R$, $y=O_2 R$, and suppose that the radius of circle $C_1$ is $r$ and the radius of circle $C_2$ is $\tfrac{5}{8}r$.

Then the power of point $R$ with respect to $C_1$ is

\[(r+x)(r-x) = r^2 - x^2 = 24^2\]

and the power of point $R$ with respect to $C_2$ is

\[\left(\frac{5}{8}r + y\right) \left(\frac{5}{8}r - y\right) = \frac{25}{64}r^2 - y^2 = 24^2.\]

Also, note that $x-y=39$.

Subtract the above two equations to find that $\tfrac{39}{64}r^2 - x^2 + y^2 = 0$ or $39 r^2 = 64(x^2-y^2)$. As $x-y=39$, we find that $r^2=64(x+y) = 64(2y+39)$. Plug this into an earlier equation to find that $25(2y+39)-y^2=24^2$. This is a quadratic equation with solutions $y=\tfrac{50 \pm 64}{2}$, and as $y$ is a length, it is positive, hence $y=57$, and $x=y+39=96$. This is the only possibility if the two centers lie on the same same of their radical axis.

On the other hand, if they lie on opposite sides, then it is clear that there is only one possibility, and then it is clear that $O_1 R + O_2 R = O_1 O_2 = 39$. Therefore, we obtain exactly four possible centers, and the sum of the desired lengths is $57+96+39 = \boxed{\textbf{(D)}\; 192}$.

Number-Intensive Solution

Start by drawing $PQ$ first, because trying to get all four circles down will take you a few years. Next, because all circles have $P$ and $Q$ on them, and since all points on a circle are equidistant from the center, all circle centers lie on the perpendicular bisector of $PQ$, and point $R$ is on this bisector.

In order for all the circle radii to be different (because the circles can't be congruent), two circle centers are on the same side of $PQ$, and two are straddling it. For the latter two circles- just call them $A$ and $B$- clearly $AR+BR$ is 39.

Now, let's take the next case. Then $C$ and $D$ lie on the same side. Construct the triangles from your picture, and use the Pythagorean Theorem (maybe divide all lengths by 3 to lessen big numbers) and then you get that the distance from $R$ to the closest circle center is $57$. Therefore, the answer is $39+2*57+39=192 \boxed{(D)}$.

See Also

2015 AMC 12B (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 12 Problems and Solutions

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