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Difference between revisions of "2017 AMC 12A Problems/Problem 16"
Line 21: Line 21:
  
 
==Solution==
 
==Solution==
 +
Connect the centers of the tangent circles! (call the center of the large circle <math>C</math>)
 +
 +
<asy>
 +
size(5cm);
 +
draw(arc((0,0),3,0,180));
 +
draw(arc((2,0),1,0,180));
 +
draw(arc((-1,0),2,0,180));
 +
draw((-3,0)--(3,0));
 +
pair P = (9/7,12/7);
 +
draw(circle(P,6/7));
 +
dot((-1,0)); dot((2,0)); dot((0,0)); dot(P);
 +
draw((-1,0)--P);
 +
draw((2,0)--P);
 +
draw((0,0)--(9/5,12/5));
 +
</asy>
 +
 +
Notice that we don't even need the circles anymore; thus, draw triangle <math>\Delta ABP</math> with cevian <math>PC</math>:
 +
 +
<asy>
 +
size(5cm);
 +
draw((-1,0)--(2,0));
 +
pair P = (9/7,12/7);
 +
dot((-1,0)); dot((2,0)); dot((0,0)); dot(P);
 +
draw((-1,0)--P);
 +
draw((2,0)--P);
 +
draw((0,0)--P);
 +
</asy>
 +
 +
and use [[Stewart's Theorem]]:
 +
 +
<cmath>AB \cdot AC \cdot BC + AB \cdot {CP}^2 = AC \cdot {BP}^2 + BC \cdot {AP}^2</cmath>
 +
 +
From what we learned from the tangent circles, we have <math>AB = 3</math>, <math>AC = 1</math>, <math>BC = 2</math>, <math>AP = 2 + r</math>, <math>BP = 1 + r</math>, and <math>CP = 3 - r</math>, where <math>r</math> is the radius of the circle centered at <math>P</math> that we seek.
 +
 +
Thus:
 +
 +
<cmath>3 \cdot 1 \cdot 2 + 3 {\left(3-r\right)}^2 = 1 {\left(1+r\right)}^2 + 2 {\left(2+r\right)}^2</cmath>
 +
<cmath>6 + 3\left(9 - 6r + r^2\right) = \left(1 + 2r + r^2\right) + 2\left(4 + 4r + r^2\right)</cmath>
 +
<cmath>33 - 18r + 3r^2 = 9 + 10r + 3r^2</cmath>
 +
<cmath>28r = 24</cmath>
 +
<cmath>r = \boxed{\frac{6}{7}} \to \boxed{\textbf{(B)}}</cmath>
  
 
==See Also==
 
==See Also==
 
{{AMC12 box|year=2017|ab=A|num-b=15|num-a=17}}
 
{{AMC12 box|year=2017|ab=A|num-b=15|num-a=17}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Revision as of 19:19, 8 February 2017

Problem

In the figure below, semicircles with centers at $A$ and $B$ and with radii 2 and 1, respectively, are drawn in the interior of, and sharing bases with, a semicircle with diameter $JK$. The two smaller semicircles are externally tangent to each other and internally tangent to the largest semicircle. A circle centered at $P$ is drawn externally tangent to the two smaller semicircles and internally tangent to the largest semicircle. What is the radius of the circle centered at $P$?

[asy] size(5cm); draw(arc((0,0),3,0,180)); draw(arc((2,0),1,0,180)); draw(arc((-1,0),2,0,180)); draw((-3,0)--(3,0)); pair P = (-1,0)+(2+6/7)*dir(36.86989); draw(circle(P,6/7)); dot((-1,0)); dot((2,0)); dot(P); [/asy]

$\textbf{(A)}\ \frac{3}{4} \qquad \textbf{(B)}\ \frac{6}{7} \qquad\textbf{(C)}\ \frac{1}{2}\sqrt{3} \qquad\textbf{(D)}\ \frac{5}{8}\sqrt{2} \qquad\textbf{(E)}\ \frac{11}{12}$

Solution

Connect the centers of the tangent circles! (call the center of the large circle $C$)

[asy] size(5cm); draw(arc((0,0),3,0,180)); draw(arc((2,0),1,0,180)); draw(arc((-1,0),2,0,180)); draw((-3,0)--(3,0)); pair P = (9/7,12/7); draw(circle(P,6/7)); dot((-1,0)); dot((2,0)); dot((0,0)); dot(P); draw((-1,0)--P); draw((2,0)--P); draw((0,0)--(9/5,12/5)); [/asy]

Notice that we don't even need the circles anymore; thus, draw triangle $\Delta ABP$ with cevian $PC$:

[asy] size(5cm); draw((-1,0)--(2,0)); pair P = (9/7,12/7); dot((-1,0)); dot((2,0)); dot((0,0)); dot(P); draw((-1,0)--P); draw((2,0)--P); draw((0,0)--P); [/asy]

and use Stewart's Theorem:

\[AB \cdot AC \cdot BC + AB \cdot {CP}^2 = AC \cdot {BP}^2 + BC \cdot {AP}^2\]

From what we learned from the tangent circles, we have $AB = 3$, $AC = 1$, $BC = 2$, $AP = 2 + r$, $BP = 1 + r$, and $CP = 3 - r$, where $r$ is the radius of the circle centered at $P$ that we seek.

Thus:

\[3 \cdot 1 \cdot 2 + 3 {\left(3-r\right)}^2 = 1 {\left(1+r\right)}^2 + 2 {\left(2+r\right)}^2\] \[6 + 3\left(9 - 6r + r^2\right) = \left(1 + 2r + r^2\right) + 2\left(4 + 4r + r^2\right)\] \[33 - 18r + 3r^2 = 9 + 10r + 3r^2\] \[28r = 24\] \[r = \boxed{\frac{6}{7}} \to \boxed{\textbf{(B)}}\]

See Also

2017 AMC 12A (Problems • Answer Key • Resources)
Preceded by
Problem 15 		Followed by
Problem 17
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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