Art of Problem Solving
AoPS Online
Math texts, online classes, and more
for students in grades 5-12.
Visit AoPS Online ‚
Books for Grades 5-12 Online Courses
Beast Academy
Engaging math books and online learning
for students ages 6-13.
Visit Beast Academy ‚
Books for Ages 6-13 Beast Academy Online
AoPS Academy
Small live classes for advanced math
and language arts learners in grades 2-12.
Visit AoPS Academy ‚
Find a Physical Campus Visit the Virtual Campus

Difference between revisions of "2020 AMC 10B Problems/Problem 14"

(→‎Video Solution)
(→‎Solution 3)
 
(11 intermediate revisions by 8 users not shown)
Line 4: Line 4:
 
As shown in the figure below, six semicircles lie in the interior of a regular hexagon with side length 2 so that the diameters of the semicircles coincide with the sides of the hexagon. What is the area of the shaded region ---- inside the hexagon but outside all of the semicircles?
 
As shown in the figure below, six semicircles lie in the interior of a regular hexagon with side length 2 so that the diameters of the semicircles coincide with the sides of the hexagon. What is the area of the shaded region ---- inside the hexagon but outside all of the semicircles?
  
−
<math>\textbf{(A) } 6\sqrt3 - 3\pi \qquad \textbf{(B) } \frac{9\sqrt3}{2} - 2\pi \qquad \textbf{(C) } \frac{3\sqrt3}{2} - \frac{3\pi}{3} \qquad \textbf{(D) } 3\sqrt3 - \pi \qquad \textbf{(E) } \frac{9\sqrt3}{2} - \pi</math>
+
<math>\textbf{(A) } 6\sqrt3 - 3\pi \qquad \textbf{(B) } \frac{9\sqrt3}{2} - 2\pi \qquad \textbf{(C) } \frac{3\sqrt3}{2} - \frac{\pi}{3} \qquad \textbf{(D) } 3\sqrt3 - \pi \qquad \textbf{(E) } \frac{9\sqrt3}{2} - \pi</math>
 
<asy>
 
<asy>
  
Line 76: Line 76:
 
</asy>
 
</asy>
  
−
Let point <math>A</math> be a vertex of the regular hexagon, let point <math>B</math> be the midpoint of the line connecting point <math>A</math> and a neighboring vertex, and let point <math>C</math> be the second intersection of the two semicircles that pass through point <math>A</math>. Then, <math>BC = 1</math>, since <math>B</math> is the center of the semicircle with radius <math>1</math> that <math>C</math> lies on, <math>AB = 1</math>, since <math>B</math> is the center of the semicircle with radius <math>1</math> that <math>A</math> lies on, and <math>\angle BAC = 60^\circ</math>, as a regular hexagon has angles of 120<math>^\circ</math>, and <math>\angle BAC</math> is half of any angle in this hexagon. Now, using the Law of Sines, <math>\frac{1}{\sin \angle ACB} = \frac{1}{\sin 60^\circ}</math>, so <math>\angle ACB = 60^\circ</math>. Since the angles in a triangle sum to 180<math>^\circ</math>, <math>\angle ABC</math> is also 60<math>^\circ</math>. Therefore, <math>\triangle ABC</math> is an equilateral triangle with side lengths of <math>1</math>.
+
Let point <math>A</math> be a vertex of the regular hexagon, let point <math>B</math> be the midpoint of the line connecting point <math>A</math> and a neighboring vertex, and let point <math>C</math> be the second intersection of the two semicircles that pass through point <math>A</math>. Then, <math>BC = 1</math>, since <math>B</math> is the center of the semicircle with radius <math>1</math> that <math>C</math> lies on, <math>AB = 1</math>, since <math>B</math> is the center of the semicircle with radius <math>1</math> that <math>A</math> lies on, and <math>\angle BAC = 60^\circ</math>, as a regular hexagon has angles of 120<math>^\circ</math>, and <math>\angle BAC</math> is half of any angle in this hexagon. Now, since <math>AB = BC</math>, <math>\angle ACB = \angle BAC = 60^\circ</math>. Since the angles in a triangle sum to 180<math>^\circ</math>, <math>\angle ABC</math> is also 60<math>^\circ</math>. Therefore, <math>\triangle ABC</math> is an equilateral triangle with side lengths of <math>1</math>.
  
 
<asy>
 
<asy>
Line 138: Line 138:
 
</asy>
 
</asy>
  
−
Since the area of a regular hexagon can be found with the formula <math>\frac{3\sqrt{3}s^2}{2}</math>, where <math>s</math> is the side length of the hexagon, the area of this hexagon is <math>\frac{3\sqrt{3}(2^2)}{2} = 6\sqrt{3}</math>. Since the area of an equilateral triangle can be found with the formula <math>\frac{\sqrt{3}}{4}s^2</math>, where <math>s</math> is the side length of the equilateral triangle, the area of an equilateral triangle with side lengths of <math>1</math> is <math>\frac{\sqrt{3}}{4}(1^2) = \frac{\sqrt{3}}{4}</math>. Since the area of a circle can be found with the formula <math>\pi r^2</math>, the area of a sixth of a circle with radius <math>1</math> is <math>\frac{\pi(1^2)}{6} = \frac{\pi}{6}</math>. In each sixth of the hexagon, there are two equilateral triangles colored white, each with an area of <math>\frac{\sqrt{3}}{4}</math>, and one-sixth of a circle with radius <math>1</math> colored white, with an area of <math>\frac{\pi}{6}</math>. The rest of the sixth is colored gray. Therefore, the total area that is colored white in each sixth of the hexagon is <math>2(\frac{\sqrt{3}}{4}) + \frac{\pi}{6}</math>, which equals <math>\frac{\sqrt{3}}{2} + \frac{\pi}{6}</math>, and the total area colored white is <math>6(\frac{\sqrt{3}}{2} + \frac{\pi}{6})</math>, which equals <math>3\sqrt{3} + \pi</math>. Since the area colored gray equals the total area of the hexagon minus the area colored white, the area colored gray is <math>6\sqrt{3} - (3\sqrt{3} + \pi)</math>, which equals <math>\boxed{\textbf{(D) }3\sqrt{3} - \pi}</math>.
+
Since the area of a regular hexagon can be found with the formula <math>\frac{3\sqrt{3}s^2}{2}</math>, where <math>s</math> is the side length of the hexagon, the area of this hexagon is <math>\frac{3\sqrt{3}(2^2)}{2} = 6\sqrt{3}</math>. Since the area of an equilateral triangle can be found with the formula <math>\frac{\sqrt{3}}{4}s^2</math>, where <math>s</math> is the side length of the equilateral triangle, the area of an equilateral triangle with side lengths of <math>1</math> is <math>\frac{\sqrt{3}}{4}\left(1^2\right) = \frac{\sqrt{3}}{4}</math>. Since the area of a circle can be found with the formula <math>\pi r^2</math>, the area of a sixth of a circle with radius <math>1</math> is <math>\frac{\pi(1^2)}{6} = \frac{\pi}{6}</math>. In each sixth of the hexagon, there are two equilateral triangles colored white, each with an area of <math>\frac{\sqrt{3}}{4}</math>, and one-sixth of a circle with radius <math>1</math> colored white, with an area of <math>\frac{\pi}{6}</math>. The rest of the sixth is colored gray. Therefore, the total area that is colored white in each sixth of the hexagon is <math>2\left(\frac{\sqrt{3}}{4}\right) + \frac{\pi}{6}</math>, which equals <math>\frac{\sqrt{3}}{2} + \frac{\pi}{6}</math>, and the total area colored white is <math>6\left(\frac{\sqrt{3}}{2} + \frac{\pi}{6}\right)</math>, which equals <math>3\sqrt{3} + \pi</math>. Since the area colored gray equals the total area of the hexagon minus the area colored white, the area colored gray is <math>6\sqrt{3} - (3\sqrt{3} + \pi)</math>, which equals <math>\boxed{\textbf{(D) }3\sqrt{3} - \pi}</math>.
  
 
==Solution 2==
 
==Solution 2==
Line 162: Line 162:
 
draw((2.5,3sqrt(3)/2)--(1.5,3sqrt(3)/2));
 
draw((2.5,3sqrt(3)/2)--(1.5,3sqrt(3)/2));
 
</asy>
 
</asy>
−
The entire rhombus is just 2 equilatrial triangles with side lengths of 1, so it has an area of:
+
The entire rhombus is just 2 equilateral triangles with side lengths of 1, so it has an area of:
 
<cmath> 2\cdot\frac{\sqrt{3}}{4}=\frac{\sqrt{3}}{2}</cmath>
 
<cmath> 2\cdot\frac{\sqrt{3}}{4}=\frac{\sqrt{3}}{2}</cmath>
−
The arc that is not included has an area of:
+
The sector that is not included has an area of:
 
<cmath> \frac16 \cdot\pi \cdot1^2 = \frac{\pi}{6}</cmath>
 
<cmath> \frac16 \cdot\pi \cdot1^2 = \frac{\pi}{6}</cmath>
 
Hence, the area of the shaded region in that section is <cmath>\frac{\sqrt{3}}{2}-\frac{\pi}{6}</cmath>
 
Hence, the area of the shaded region in that section is <cmath>\frac{\sqrt{3}}{2}-\frac{\pi}{6}</cmath>
Line 175: Line 175:
 
Then the area of the shaded region can be expressed in the following two ways:
 
Then the area of the shaded region can be expressed in the following two ways:
  
−
<math>[1]</math>. <math>Area</math> of hexagon - (<math>Area</math> of six semicircles <math>-</math> <math>Area</math> of six <math>old</math> "leaves") <math>=</math> <math>\frac{\sqrt{3}}{4}*4*6 - (3\pi - S);</math>
+
<math>[1]</math>. <math>\textbf{Area}</math> of hexagon - (<math>\textbf{Area}</math> of six semicircles <math>-</math> <math>\textbf{Area}</math> of six <math>\textbf{old}</math> "leaves") <math>=</math> <math>\frac{\sqrt{3}}{4} \cdot 4 \cdot 6 - (3\pi - S);</math>
  
−
<math>[2]</math>. <math>Area</math>  of  dashed  circle <math>-</math> <math>Area</math> of six <math>new</math> "leaves" = <math>\pi - S;</math>
+
<math>[2]</math>. <math>\textbf{Area}</math>  of  dashed  circle <math>-</math> <math>\textbf{Area}</math> of six <math>\textbf{new}</math> "leaves" = <math>\pi - S;</math>
  
−
Then we have <math>\frac{\sqrt{3}}{4}*4*6 - (3\pi - S) = \pi - S</math>     
+
Then we have <math>\frac{\sqrt{3}}{4} \cdot 4 \cdot 6 - (3\pi - S) = \pi - S</math>     
  
 
<math>S = 2\pi - 3\sqrt{3}</math>
 
<math>S = 2\pi - 3\sqrt{3}</math>
Line 188: Line 188:
  
 
--Extremelysupercooldude (Minor latex and grammar edits)
 
--Extremelysupercooldude (Minor latex and grammar edits)
 +
 +
~Yelechi (LaTeX edits)
  
 
<asy>
 
<asy>
Line 278: Line 280:
 
~MrThinker
 
~MrThinker
  
−
==Video Solution==
+
==Solution 5==
−
https://youtu.be/t6yjfKXpwDs?t=786 (for AMC 10)
+
<asy>
 +
size(140);
 +
fill((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--cycle,gray(0.4));
 +
fill(arc((2,0),1,180,0)--(2,0)--cycle,white);
 +
fill(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle,white);
 +
fill(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle,white);
 +
fill(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle,white);
 +
fill(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle,white);
 +
fill(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle,white);
 +
draw((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--(1,0));
 +
label("$2$",(3.5,3sqrt(3)/2),NE);
 +
draw(arc((2,0),1,180,0)--(2,0)--cycle);
 +
draw(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle);
 +
draw(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle);
 +
draw(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle);
 +
draw(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle);
 +
draw(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle);
 +
draw((2.5,3sqrt(3)/2)--(1.5,3sqrt(3)/2));
 +
draw((2.5,0.87)--(3,2sqrt(3)/2));
 +
draw((3,2sqrt(3)/2)--(2.5,3sqrt(3)/2));
 +
draw((1,1.73)--(1.5,2.6));
 +
draw((1.5,0.87)--(1,1.73));
 +
draw((2.5,sqrt(3)/2)--(1.5,sqrt(3)/2));
 +
label("$1$", (2, 0.9), S);
 +
draw((1.43, 1)--(2, 0)--(2.57, 1));
 +
</asy>
 +
 
 +
First we connect every vertex of the shaded portion to another, forming an equilateral hexagon. Each interior angle of this hexagon is <math>120^{\circ},</math> the interior angles of the new triangle are all <math>60^{\circ}</math>. As the radius of the semicircle is <math>1</math>, we know that this hexagon has side length <math>1</math>, so its area is given by the combined area of six equilateral triangles with side length <math>1</math>, or <math>\frac{\sqrt{3}}{4}\cdot 6 = \frac{3\sqrt{3}}{2}.</math>
 +
 
 +
Now, we have to subtract off the six mini-sectors between the triangle and the shaded area. The area of one of these mini-sectors is the difference between a <math>60^{\circ}</math> sector of the semicircle and the equilateral we just drew. The area of a sector is just <math>\frac{\pi}{6}</math>, and the area of a triangle is <math>\frac{\sqrt{3}}{4}.</math> Therefore our combined area of the mini-sectors is
 +
<cmath>6\left(\frac{\pi}{6} - \frac{3\sqrt{3}}{2}\right) = \pi-\frac{3\sqrt{3}}{2}.</cmath>
 +
Subtracting this from the area of the (smaller) hexagon, the desired area is
 +
 
 +
<cmath>\frac{3\sqrt{3}}{2} -\left(\pi - \frac{3\sqrt{3}}{2}\right) = \boxed{\textbf{(D)} ~3\sqrt{3} - \pi}</cmath>
 +
 
 +
-Benedict T (countmath1)
 +
 
 +
==Video Solution (HOW TO CREATIVELY THINK AND SOLVE!!!)==
 +
https://youtu.be/LT2TQj9Y6KI
 +
 
 +
~Education, the Study of Everything
 +
 
  
−
https://youtu.be/0xgTR3UEqbQ (for AMC 12)
 
  
−
~IceMatrix
+
 
 +
==Video Solution==
 +
https://youtu.be/t6yjfKXpwDs?t=786
 +
https://youtu.be/0xgTR3UEqbQ
  
 
==Video Solution==
 
==Video Solution==
Line 290: Line 335:
 
~DSA_Catachu
 
~DSA_Catachu
  
−
== Video Solution by OmegaLearn ==
+
== Video Solution ==
 
https://youtu.be/NsQbhYfGh1Q?t=2046
 
https://youtu.be/NsQbhYfGh1Q?t=2046
−
 
−
~ pi_is_3.14
 
−
 
−
==Video Solution==
 
−
https://youtu.be/LT2TQj9Y6KI
 
−
 
−
~Education, the Study of Everything
 
  
 
==See Also==
 
==See Also==

Latest revision as of 20:05, 3 November 2024

The following problem is from both the 2020 AMC 10B #14 and 2020 AMC 12B #11, so both problems redirect to this page.

Problem

As shown in the figure below, six semicircles lie in the interior of a regular hexagon with side length 2 so that the diameters of the semicircles coincide with the sides of the hexagon. What is the area of the shaded region ---- inside the hexagon but outside all of the semicircles?

$\textbf{(A) } 6\sqrt3 - 3\pi \qquad \textbf{(B) } \frac{9\sqrt3}{2} - 2\pi \qquad \textbf{(C) } \frac{3\sqrt3}{2} - \frac{\pi}{3} \qquad \textbf{(D) } 3\sqrt3 - \pi \qquad \textbf{(E) } \frac{9\sqrt3}{2} - \pi$ [asy] size(140); fill((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--cycle,gray(0.4)); fill(arc((2,0),1,180,0)--(2,0)--cycle,white); fill(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle,white); fill(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle,white); fill(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle,white); fill(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle,white); fill(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle,white); draw((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--(1,0)); draw(arc((2,0),1,180,0)--(2,0)--cycle); draw(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle); draw(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle); draw(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle); draw(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle); draw(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle); label("$2$",(3.5,3sqrt(3)/2),NE); [/asy]

Solution 1

[asy] real x=sqrt(3); real y=2sqrt(3); real z=3.5; real a=x/2; real b=0.5; real c=3a; pair A, B, C, D, E, F; A = (1,0); B = (3,0); C = (4,x); D = (3,y); E = (1,y); F = (0,x); fill(A--B--C--D--E--F--A--cycle,grey); fill(arc((2,0),1,0,180)--cycle,white); fill(arc((2,y),1,180,360)--cycle,white); fill(arc((z,a),1,60,240)--cycle,white); fill(arc((b,a),1,300,480)--cycle,white); fill(arc((b,c),1,240,420)--cycle,white); fill(arc((z,c),1,120,300)--cycle,white); draw(A--B--C--D--E--F--A); draw(arc((z,c),1,120,300)); draw(arc((b,c),1,240,420)); draw(arc((b,a),1,300,480)); draw(arc((z,a),1,60,240)); draw(arc((2,y),1,180,360)); draw(arc((2,0),1,0,180)); label("2",(z,c),NE); pair X,Y,Z; X = (1,0); Y = (2,0); Z = (1.5,a); pair d = 1.9*dir(7); dot(X); dot(Y); dot(Z); label("A",X,SW); label("B",Y,SE); label("C",Z,N); draw(X--Y--Z--A); label("1",(1.5,0),S); label("1",(1.75,a/2),dir(30)); draw(anglemark(Y,X,Z,8),blue); label("$60^\circ$",anglemark(Y,X,Z),d); [/asy]

Let point $A$ be a vertex of the regular hexagon, let point $B$ be the midpoint of the line connecting point $A$ and a neighboring vertex, and let point $C$ be the second intersection of the two semicircles that pass through point $A$. Then, $BC = 1$, since $B$ is the center of the semicircle with radius $1$ that $C$ lies on, $AB = 1$, since $B$ is the center of the semicircle with radius $1$ that $A$ lies on, and $\angle BAC = 60^\circ$, as a regular hexagon has angles of 120$^\circ$, and $\angle BAC$ is half of any angle in this hexagon. Now, since $AB = BC$, $\angle ACB = \angle BAC = 60^\circ$. Since the angles in a triangle sum to 180$^\circ$, $\angle ABC$ is also 60$^\circ$. Therefore, $\triangle ABC$ is an equilateral triangle with side lengths of $1$.

[asy] real x=sqrt(3); real y=2sqrt(3); real z=3.5; real a=x/2; real b=0.5; real c=3a; pair A, B, C, D, E, F; A = (1,0); B = (3,0); C = (4,x); D = (3,y); E = (1,y); F = (0,x); fill(A--B--C--D--E--F--A--cycle,grey); fill(arc((2,0),1,0,180)--cycle,white); fill(arc((2,y),1,180,360)--cycle,white); fill(arc((z,a),1,60,240)--cycle,white); fill(arc((b,a),1,300,480)--cycle,white); fill(arc((b,c),1,240,420)--cycle,white); fill(arc((z,c),1,120,300)--cycle,white); draw(A--B--C--D--E--F--A); draw(arc((z,c),1,120,300)); draw(arc((b,c),1,240,420)); draw(arc((b,a),1,300,480)); draw(arc((z,a),1,60,240)); draw(arc((2,y),1,180,360)); draw(arc((2,0),1,0,180)); pair G,H,I,J,K; G = (2,0); H = (2.5,a); I = (1.5,a); J = (1,0); K = (3,0); pair d = 2.7*dir(78); dot(G); dot(H); dot(I); dot(J); dot(K); label("2",(z,c),NE); label("1",(1.5,0),S); label("1",(2.5,0),S); add(pathticks(G--J,1,0.5,0,3,red)); add(pathticks(I--J,1,0.5,0,3,red)); add(pathticks(G--I,1,0.5,0,3,red)); add(pathticks(G--H,1,0.5,0,3,red)); add(pathticks(G--K,1,0.5,0,3,red)); add(pathticks(K--H,1,0.5,0,3,red)); label("$60^\circ$",anglemark(H,G,I),d); draw(anglemark(H,G,I,8),blue); label("$60^\circ$",G,2*dir(146)); draw(anglemark(I,G,J,8),blue); label("$60^\circ$",G,2.8*dir(28)); draw(anglemark(K,G,H,8),blue); draw(G--J--I--G); draw(G--H--K--G); [/asy]

Since the area of a regular hexagon can be found with the formula $\frac{3\sqrt{3}s^2}{2}$, where $s$ is the side length of the hexagon, the area of this hexagon is $\frac{3\sqrt{3}(2^2)}{2} = 6\sqrt{3}$. Since the area of an equilateral triangle can be found with the formula $\frac{\sqrt{3}}{4}s^2$, where $s$ is the side length of the equilateral triangle, the area of an equilateral triangle with side lengths of $1$ is $\frac{\sqrt{3}}{4}\left(1^2\right) = \frac{\sqrt{3}}{4}$. Since the area of a circle can be found with the formula $\pi r^2$, the area of a sixth of a circle with radius $1$ is $\frac{\pi(1^2)}{6} = \frac{\pi}{6}$. In each sixth of the hexagon, there are two equilateral triangles colored white, each with an area of $\frac{\sqrt{3}}{4}$, and one-sixth of a circle with radius $1$ colored white, with an area of $\frac{\pi}{6}$. The rest of the sixth is colored gray. Therefore, the total area that is colored white in each sixth of the hexagon is $2\left(\frac{\sqrt{3}}{4}\right) + \frac{\pi}{6}$, which equals $\frac{\sqrt{3}}{2} + \frac{\pi}{6}$, and the total area colored white is $6\left(\frac{\sqrt{3}}{2} + \frac{\pi}{6}\right)$, which equals $3\sqrt{3} + \pi$. Since the area colored gray equals the total area of the hexagon minus the area colored white, the area colored gray is $6\sqrt{3} - (3\sqrt{3} + \pi)$, which equals $\boxed{\textbf{(D) }3\sqrt{3} - \pi}$.

Solution 2

First, subdivide the hexagon into 24 equilateral triangles with side length 1: [asy] size(140); fill((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--cycle,gray(0.4)); fill(arc((2,0),1,180,0)--(2,0)--cycle,white); fill(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle,white); fill(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle,white); fill(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle,white); fill(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle,white); fill(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle,white); draw((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--(1,0)); draw(arc((2,0),1,180,0)--(2,0)--cycle); draw(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle); draw(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle); draw(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle); draw(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle); draw(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle); label("$2$",(3.5,3sqrt(3)/2),NE); draw((1,0)--(3,2sqrt(3))); draw((3,0)--(1,2sqrt(3))); draw((4,sqrt(3))--(0,sqrt(3))); draw((2,0)--(3.5,3sqrt(3)/2)); draw((3.5,sqrt(3)/2)--(2,2sqrt(3))); draw((3.5,3sqrt(3)/2)--(0.5,3sqrt(3)/2)); draw((2,2sqrt(3))--(0.5,sqrt(3)/2)); draw((2,0)--(0.5,3sqrt(3)/2)); draw((3.5,sqrt(3)/2)--(0.5,sqrt(3)/2)); [/asy] Now note that the entire shaded region is just 6 times this part: [asy] size(100); fill((2,sqrt(3))--(2.5,3sqrt(3)/2)--(2,2sqrt(3))--(1.5,3sqrt(3)/2)--cycle,gray(0.4)); fill(arc((2,2sqrt(3)),1,240,300)--(2,2sqrt(3))--cycle,white); draw(arc((2,2sqrt(3)),1,240,300)--(2,2sqrt(3))--cycle); label("$1$",(2.25,7sqrt(3)/4),NE); draw((2,sqrt(3))--(2.5,3sqrt(3)/2)--(2,2sqrt(3))--(1.5,3sqrt(3)/2)--cycle); draw((2.5,3sqrt(3)/2)--(1.5,3sqrt(3)/2)); [/asy] The entire rhombus is just 2 equilateral triangles with side lengths of 1, so it has an area of: \[2\cdot\frac{\sqrt{3}}{4}=\frac{\sqrt{3}}{2}\] The sector that is not included has an area of: \[\frac16 \cdot\pi \cdot1^2 = \frac{\pi}{6}\] Hence, the area of the shaded region in that section is \[\frac{\sqrt{3}}{2}-\frac{\pi}{6}\] For a final area of: \[6\left(\frac{\sqrt{3}}{2}-\frac{\pi}{6}\right)=3\sqrt{3}-\pi\Rightarrow \boxed{\mathrm{(D)}}\] ~N828335

Solution 3

We can see there are six congruent "leaves" on the original figure, and let us say the total area of them is $S$. If we make a circle with radius one in the circle (the dashed one), we can get another six congruent "leaves", and they are the same as the old six ones. So the area of the six new leaves can also be represented by $S$. Then the area of the shaded region can be expressed in the following two ways:

$[1]$. $\textbf{Area}$ of hexagon - ($\textbf{Area}$ of six semicircles $-$ $\textbf{Area}$ of six $\textbf{old}$ "leaves") $=$ $\frac{\sqrt{3}}{4} \cdot 4 \cdot 6 - (3\pi - S);$

$[2]$. $\textbf{Area}$ of dashed circle $-$ $\textbf{Area}$ of six $\textbf{new}$ "leaves" = $\pi - S;$

Then we have $\frac{\sqrt{3}}{4} \cdot 4 \cdot 6 - (3\pi - S) = \pi - S$

$S = 2\pi - 3\sqrt{3}$

Plugging $S$ back in to either $[1]$ or $[2]$, we can solve for the shaded area $= 3\sqrt{3} - \pi$ $\rightarrow\boxed{\mathrm{(D)}}$

--RyanZ@BRS

--Extremelysupercooldude (Minor latex and grammar edits)

~Yelechi (LaTeX edits)

[asy] size(140); fill((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--cycle,gray(0.4)); fill(arc((2,0),1,180,0)--(2,0)--cycle,white); fill(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle,white); fill(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle,white); fill(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle,white); fill(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle,white); fill(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle,white); draw((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--(1,0)); draw(arc((2,0),1,180,0)--(2,0)--cycle); draw(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle); draw(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle); draw(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle); draw(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle); draw(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle); label("$2$",(3.5,3sqrt(3)/2),NE); draw(Arc((2,sqrt(3)), 1, 0, 360),dashed+linewidth(.5)); [/asy]

Solution 4 (Variables)

We can set several variables.

Let $x$ be the area of the shaded region, and let $y$ be the area of one of the 'leaf-shaped' regions.

[asy] size(140); fill((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--cycle,gray(0.4)); fill(arc((2,0),1,180,0)--(2,0)--cycle,white); fill(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle,white); fill(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle,white); fill(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle,white); fill(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle,white); fill(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle,white); draw((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--(1,0)); label("$2$",(3.5,3sqrt(3)/2),NE); label("$x$",(2,1.6),N); label("$y$",(1.3,3.2),S); draw(arc((2,0),1,180,0)--(2,0)--cycle); draw(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle); draw(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle); draw(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle); draw(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle); draw(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle); draw((2.5,3sqrt(3)/2)--(1.5,3sqrt(3)/2)); draw((2.5,0.87)--(3,2sqrt(3)/2)); draw((3,2sqrt(3)/2)--(2.5,3sqrt(3)/2)); draw((1,1.73)--(1.5,2.6)); draw((1.5,0.87)--(1,1.73)); draw((2.5,sqrt(3)/2)--(1.5,sqrt(3)/2)); label("$1$", (1.2,2.2),W); [/asy]

We then can connect the vertices of $x$, and we end up with another hexagon, with side length $1$. Since each of the regions that are outside of $x$ but inside of the smaller hexagon is half of $y$, their total area is $\frac{6y}{2} = 3y$. The smaller hexagon minus this region would be $x$. So, \[x= \frac{3\sqrt{3}}{2} \cdot 1^{2} - 3y\]

Also, the area of the larger hexagon minus the semicircles then add back $6y$ would also be $x$. (Note that by just subtracting the semicircles from the larger hexagon would result in an overlap of $6y$.) So,

\[x= \frac{3\sqrt{3}}{2} \cdot 2^{2} - 3\pi + 6y\]

Equating the equations, we have

\begin{align*} \frac{3\sqrt{3}}{2} \cdot 1^{2} - 3y &= \frac{3\sqrt{3}}{2} \cdot 2^{2} - 3\pi + 6y \\ \frac{3\sqrt{3}}{2} - 3y &= 6\sqrt{3} - 3\pi + 6y \\ \end{align*} Solving for $y$, we have

\begin{align*} 9y &= \frac{3\sqrt{3}}{2} - 6\sqrt{3} + 3\pi \\ &=-\frac{9\sqrt{3}}{2} + 3\pi \\ \end{align*}

Then, we can plug this back into

\[x= \frac{3\sqrt{3}}{2} \cdot 1^{2} - 3y\]

Since $9y = -\frac{9\sqrt{3}}{2} + 3\pi$, dividing by $3$ gives

\begin{align*} 3y &= -\frac{9\sqrt{3}}{2} \times \frac{1}{3} + \pi \\ &=-\frac{3\sqrt{3}}{2} + \pi \\ \end{align*}

Therefore, $x = \frac{3\sqrt{3}}{2} - (-\frac{3\sqrt{3}}{2} + \pi) \implies x= \boxed{\textbf{(D)} ~3\sqrt{3} - \pi}$.

~MrThinker

Solution 5

[asy] size(140); fill((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--cycle,gray(0.4)); fill(arc((2,0),1,180,0)--(2,0)--cycle,white); fill(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle,white); fill(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle,white); fill(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle,white); fill(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle,white); fill(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle,white); draw((1,0)--(3,0)--(4,sqrt(3))--(3,2sqrt(3))--(1,2sqrt(3))--(0,sqrt(3))--(1,0)); label("$2$",(3.5,3sqrt(3)/2),NE); draw(arc((2,0),1,180,0)--(2,0)--cycle); draw(arc((3.5,sqrt(3)/2),1,60,240)--(3.5,sqrt(3)/2)--cycle); draw(arc((3.5,3sqrt(3)/2),1,120,300)--(3.5,3sqrt(3)/2)--cycle); draw(arc((2,2sqrt(3)),1,180,360)--(2,2sqrt(3))--cycle); draw(arc((0.5,3sqrt(3)/2),1,240,420)--(0.5,3sqrt(3)/2)--cycle); draw(arc((0.5,sqrt(3)/2),1,300,480)--(0.5,sqrt(3)/2)--cycle); draw((2.5,3sqrt(3)/2)--(1.5,3sqrt(3)/2)); draw((2.5,0.87)--(3,2sqrt(3)/2)); draw((3,2sqrt(3)/2)--(2.5,3sqrt(3)/2)); draw((1,1.73)--(1.5,2.6)); draw((1.5,0.87)--(1,1.73)); draw((2.5,sqrt(3)/2)--(1.5,sqrt(3)/2)); label("$1$", (2, 0.9), S); draw((1.43, 1)--(2, 0)--(2.57, 1)); [/asy]

First we connect every vertex of the shaded portion to another, forming an equilateral hexagon. Each interior angle of this hexagon is $120^{\circ},$ the interior angles of the new triangle are all $60^{\circ}$. As the radius of the semicircle is $1$, we know that this hexagon has side length $1$, so its area is given by the combined area of six equilateral triangles with side length $1$, or $\frac{\sqrt{3}}{4}\cdot 6 = \frac{3\sqrt{3}}{2}.$

Now, we have to subtract off the six mini-sectors between the triangle and the shaded area. The area of one of these mini-sectors is the difference between a $60^{\circ}$ sector of the semicircle and the equilateral we just drew. The area of a sector is just $\frac{\pi}{6}$, and the area of a triangle is $\frac{\sqrt{3}}{4}.$ Therefore our combined area of the mini-sectors is \[6\left(\frac{\pi}{6} - \frac{3\sqrt{3}}{2}\right) = \pi-\frac{3\sqrt{3}}{2}.\] Subtracting this from the area of the (smaller) hexagon, the desired area is

\[\frac{3\sqrt{3}}{2} -\left(\pi - \frac{3\sqrt{3}}{2}\right) = \boxed{\textbf{(D)} ~3\sqrt{3} - \pi}\]

-Benedict T (countmath1)

Video Solution (HOW TO CREATIVELY THINK AND SOLVE!!!)

https://youtu.be/LT2TQj9Y6KI

~Education, the Study of Everything



Video Solution

https://youtu.be/t6yjfKXpwDs?t=786 https://youtu.be/0xgTR3UEqbQ

Video Solution

https://youtu.be/oTqx8OqSMQI

~DSA_Catachu

Video Solution

https://youtu.be/NsQbhYfGh1Q?t=2046

See Also

2020 AMC 10B (Problems • Answer Key • Resources)
Preceded by
Problem 13 		Followed by
Problem 15
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 10 Problems and Solutions
2020 AMC 12B (Problems • Answer Key • Resources)
Preceded by
Problem 10 		Followed by
Problem 12
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

The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions. AMC logo.png

Retrieved from "https://artofproblemsolving.com/wiki/index.php?title=2020_AMC_10B_Problems/Problem_14&oldid=231430"