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Difference between revisions of "2022 AIME II Problems/Problem 7"
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(Taking diagram names from Solution 1. Also say the line that passes through O_1 and is parallel to line EF, call the points of intersection of that line and the circumference of circle O_1 points X and Y.)
 
(Taking diagram names from Solution 1. Also say the line that passes through O_1 and is parallel to line EF, call the points of intersection of that line and the circumference of circle O_1 points X and Y.)
  
−
First notice that DO_1 is a straight line because DXY is an isosceles triangle(or you can realize it by symmetry). That means, because DO_1 is a straight line, so angle BDO_2 = angle ADO_1, triangle ADO_1 is similar to triangle BDO_2. Also name DO_2 = x. By our similar triangles, BO_2/AO_1 = 1/4 = x/(x+30). Solving we get x = 10 = DO_2. Pythagorean Theorem on triangle DBO_2 shows BD = sqrt(10^2 - 6^2) = 8. By similar triangles, DA = 4*8 = 32 which means AB = DA - DB = 32 - 8 = 24. Because BE = CE = AE, AB = 2*BE = 24. BE = 12, which means CE = 12. CD = DO_2(its value found earlier in this solution) + CO_2(O_2 's radius) = 10 + 6 = 16. The area of DEF is 1/2 * CD * EF = CD * CE (because CE is 1/2 of EF) = 16 * 12 = 192.
+
First notice that <math>DO_1</math> is a straight line because DXY is an isosceles triangle(or you can realize it by symmetry). That means, because <math>DO_1</math> is a straight line, so angle <math>BDO_2 = \angle ADO_1</math>, triangle <math>ADO_1</math> is similar to triangle <math>BDO_2</math>. Also name <math>DO_2 = x</math>. By our similar triangles, <math>\frac{BO_2}{AO_1} = \frac{1}{4} = \frac{x}{(x+30)}</math>. Solving we get <math>x = 10 = DO_2</math>. Pythagorean Theorem on triangle <math>DBO_2</math> shows <math>BD = sqrt(10^2 - 6^2) = 8</math>. By similar triangles, <math>DA = 4*8 = 32</math> which means <math>AB = DA - DB = 32 - 8 = 24</math>. Because <math>BE = CE = AE</math>, <math>AB = 2*BE = 24</math>. <math>BE = 12</math>, which means <math>CE = 12</math>. <math>CD = DO_2 \mbox{(its value found earlier in this solution)} + CO_2\mbox{(O_2 's radius)} = 10 + 6 = 16</math>. The area of DEF is <math>\frac{1}{2} * CD * EF = CD * CE \mbox{(because CE is 1/2 of EF)} = 16 * 12 = 192</math>.
  
 
~Professor Rat's solution, added by heheman
 
~Professor Rat's solution, added by heheman

Revision as of 18:45, 29 April 2022

Problem

A circle with radius $6$ is externally tangent to a circle with radius $24$. Find the area of the triangular region bounded by the three common tangent lines of these two circles.

Solution 1

[asy] //Created by isabelchen size(12cm, 12cm); draw(circle((0,0),24)); draw(circle((30,0),6)); draw((72/5, 96/5) -- (40,0)); draw((72/5, -96/5) -- (40,0)); draw((24, 12) -- (24, -12)); draw((0, 0) -- (40, 0)); draw((72/5, 96/5) -- (0,0)); draw((168/5, 24/5) -- (30,0)); draw((54/5, 72/5) -- (30,0)); dot((72/5, 96/5)); label("$A$",(72/5, 96/5),NE); dot((168/5, 24/5)); label("$B$",(168/5, 24/5),NE); dot((24,0)); label("$C$",(24,0),NW); dot((40, 0)); label("$D$",(40, 0),NE); dot((24, 12)); label("$E$",(24, 12),NE); dot((24, -12)); label("$F$",(24, -12),SE); dot((54/5, 72/5)); label("$G$",(54/5, 72/5),NW); dot((0, 0)); label("$O_1$",(0, 0),S); dot((30, 0)); label("$O_2$",(30, 0),S); [/asy]

$r_1 = O_1A = 24$, $r_2 = O_2B = 6$, $AG = BO_2 = r_2 = 6$, $O_1G = r_1 - r_2 = 24 - 6 = 18$, $O_1O_2 = r_1 + r_2 = 30$

$\triangle O_2BD \sim \triangle O_1GO_2$, $\frac{O_2D}{O_1O_2} = \frac{BO_2}{GO_1}$, $\frac{O_2D}{30} = \frac{6}{18}$, $O_2D = 10$

$CD = O_2D + r_1 = 10 + 6 = 16$,

$EF = 2EC = EA + EB = AB = GO_2 = \sqrt{(O_1O_2)^2-(O_1G)^2} = \sqrt{30^2-18^2} = 24$

$DEF = \frac12 \cdot EF \cdot CD = \frac12 \cdot 24 \cdot 16 = \boxed{\textbf{192}}$

~isabelchen

Solution 2

Let the center of the circle with radius $6$ be labeled $A$ and the center of the circle with radius $24$ be labeled $B$. Drop perpendiculars on the same side of line $AB$ from $A$ and $B$ to each of the tangents at points $C$ and $D$, respectively. Then, let line $AB$ intersect the two diagonal tangents at point $P$. Since $\triangle{APC} \sim \triangle{BPD}$, we have \[\frac{AP}{AP+30}=\frac14 \implies AP=10.\] Next, throw everything on a coordinate plane with $A=(0, 0)$ and $B = (30, 0)$. Then, $P = (-10, 0)$, and if $C = (x, y)$, we have \[(x+10)^2+y^2=64,\] \[x^2+y^2=36.\] Combining these and solving, we get $(x, y)=\left(-\frac{18}5, \frac{24}5\right)$. Notice now that $P$, $C$, and the intersections of the lines $x=6$ (the vertical tangent) with the tangent containing these points are collinear, and thus every slope between a pair of points will have the same slope, which in this case is $\frac{-\frac{18}5+10}{\frac{24}5}=\frac34$. Thus, the other two vertices of the desired triangle are $(6, 12)$ and $(6, -12)$. By the Shoelace Formula, the area of a triangle with coordinates $(-10, 0)$, $(6, 12)$, and $(6, -12)$ is \[\frac12|-120-0-72-72+0-120|=\boxed{\textbf{192}}.\]

~A1001

Solution 3

(Taking diagram names from Solution 1. Also say the line that passes through O_1 and is parallel to line EF, call the points of intersection of that line and the circumference of circle O_1 points X and Y.)

First notice that $DO_1$ is a straight line because DXY is an isosceles triangle(or you can realize it by symmetry). That means, because $DO_1$ is a straight line, so angle $BDO_2 = \angle ADO_1$, triangle $ADO_1$ is similar to triangle $BDO_2$. Also name $DO_2 = x$. By our similar triangles, $\frac{BO_2}{AO_1} = \frac{1}{4} = \frac{x}{(x+30)}$. Solving we get $x = 10 = DO_2$. Pythagorean Theorem on triangle $DBO_2$ shows $BD = sqrt(10^2 - 6^2) = 8$. By similar triangles, $DA = 4*8 = 32$ which means $AB = DA - DB = 32 - 8 = 24$. Because $BE = CE = AE$, $AB = 2*BE = 24$. $BE = 12$, which means $CE = 12$. $CD = DO_2 \mbox{(its value found earlier in this solution)} + CO_2\mbox{(O_2 's radius)} = 10 + 6 = 16$ (Error compiling LaTeX. Unknown error_msg). The area of DEF is $\frac{1}{2} * CD * EF = CD * CE \mbox{(because CE is 1/2 of EF)} = 16 * 12 = 192$.

~Professor Rat's solution, added by heheman

Video Solution (Mathematical Dexterity)

https://www.youtube.com/watch?v=7NGkVu0kE08

See Also

2022 AIME II (Problems • Answer Key • Resources)
Preceded by
Problem 6 		Followed by
Problem 8
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15
All AIME Problems and Solutions

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