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 "2022 AMC 12B Problems/Problem 25"

(→‎Solution 1 (Coord bash))
(→‎Solution 1 (Coord bash))
Line 27: Line 27:
 
         label("$A$", (3-sqrt(3), 3-sqrt(3)), NE);
 
         label("$A$", (3-sqrt(3), 3-sqrt(3)), NE);
 
         label("$B$", (sqrt(3) - 2, 3-sqrt(3)), NW);
 
         label("$B$", (sqrt(3) - 2, 3-sqrt(3)), NW);
 +
        label("$M$", (0, sqrt(3)), NW);
 +
        label("$N$", (1, sqrt(3)), NE);
 
</asy>
 
</asy>
  
−
Let the center of the square be <math>O</math>, the place on the border where the top and the right hexagons intersect <math>A</math>, and the place on the border where the top and the left hexagons intersect <math>B</math>. Let the origin be <math>O</math>. We would like to find the coordinates of <math>A</math>.  
+
Refer to the diagram above.
  
 
By symmetry, <math>A</math> lies on the line <math>y = x</math>. The equation of the side of the top hexagon that <math>AB</math> is on is <math>y = -x\sqrt{3} + \frac{3}{2}\sqrt{3} - \frac{1}{2}</math>. Thus, we can solve for the coordinates of <math>A</math>:
 
By symmetry, <math>A</math> lies on the line <math>y = x</math>. The equation of the side of the top hexagon that <math>AB</math> is on is <math>y = -x\sqrt{3} + \frac{3}{2}\sqrt{3} - \frac{1}{2}</math>. Thus, we can solve for the coordinates of <math>A</math>:
Line 40: Line 42:
 
<cmath>\therefore x, y = \frac{5}{2} - \sqrt{3}.</cmath>
 
<cmath>\therefore x, y = \frac{5}{2} - \sqrt{3}.</cmath>
  
−
This means that we can find the length <math>AB</math>, which is equal to <math>2(\frac{5}{2} - \sqrt{3}) = (5 - 2\sqrt{3}</math>. Next, let the top points of the top hexagon be <math>M</math> and <math>N</math>. We will next find the area of trapezoid <math>ABMN</math>. The lengths of the bases are <math>1</math> and <math>5 - 2\sqrt{3}</math>, and the height is equal to the <math>y</math>-coordinate of <math>M</math> minus the <math>y</math>-coordinate of <math>A</math>. The height of the hexagon is <math>\sqrt{3}</math> and the bottom of the hexagon lies on the line <math>y = \frac{1}{2}</math>. Thus, the <math>y</math>-coordinate of <math>M</math> is <math>\sqrt{3} - \frac{1}{2}</math>, and the height is <math>2\sqrt{3} - 3</math>. We can now find the area of the trapezoid:
+
This means that we can find the length <math>AB</math>, which is equal to <math>2(\frac{5}{2} - \sqrt{3}) = (5 - 2\sqrt{3}</math>. We will next find the area of trapezoid <math>ABMN</math>. The lengths of the bases are <math>1</math> and <math>5 - 2\sqrt{3}</math>, and the height is equal to the <math>y</math>-coordinate of <math>M</math> minus the <math>y</math>-coordinate of <math>A</math>. The height of the hexagon is <math>\sqrt{3}</math> and the bottom of the hexagon lies on the line <math>y = \frac{1}{2}</math>. Thus, the <math>y</math>-coordinate of <math>M</math> is <math>\sqrt{3} - \frac{1}{2}</math>, and the height is <math>2\sqrt{3} - 3</math>. We can now find the area of the trapezoid:
 
<cmath>[ABMN] = (2\sqrt{3} - 3)\left(\frac{1 + 5 - 2\sqrt{3}}{2}\right)</cmath>
 
<cmath>[ABMN] = (2\sqrt{3} - 3)\left(\frac{1 + 5 - 2\sqrt{3}}{2}\right)</cmath>
 
<cmath> = (2\sqrt{3} - 3)(3 - \sqrt{3})</cmath>
 
<cmath> = (2\sqrt{3} - 3)(3 - \sqrt{3})</cmath>

Revision as of 12:33, 20 November 2022

Problem

Four regular hexagons surround a square with side length 1, each one sharing an edge with the square, as shown in the figure below. The area of the resulting 12-sided outer nonconvex polygon can be written as $m \sqrt{n} + p$, where $m$, $n$, and $p$ are integers and $n$ is not divisible by the square of any prime. What is $m+n+p$?

$\textbf{(A) } -12 \qquad \textbf{(B) }-4 \qquad \textbf{(C) } 4 \qquad \textbf{(D) }24 \qquad \textbf{(E) }32$

Solution 1 (Coord bash)

[asy] import geometry; unitsize(3cm); draw((0,0) -- (1,0) -- (1,1) -- (0,1) -- cycle); draw(shift((1/2,1-sqrt(3)/2))*polygon(6)); draw(shift((1/2,sqrt(3)/2))*polygon(6)); draw(shift((sqrt(3)/2,1/2))*rotate(90)*polygon(6)); draw(shift((1-sqrt(3)/2,1/2))*rotate(90)*polygon(6)); draw((0,1-sqrt(3))--(1,1-sqrt(3))--(3-sqrt(3),sqrt(3)-2)--(sqrt(3),0)--(sqrt(3),1)--(3-sqrt(3),3-sqrt(3))--(1,sqrt(3))--(0,sqrt(3))--(sqrt(3)-2,3-sqrt(3))--(1-sqrt(3),1)--(1-sqrt(3),0)--(sqrt(3)-2,sqrt(3)-2)--cycle,linewidth(2)); label("$O (0, 0)$",(0.5,0.5),S); dot((0.5,0.5)); label("$A$", (3-sqrt(3), 3-sqrt(3)), NE); label("$B$", (sqrt(3) - 2, 3-sqrt(3)), NW); label("$M$", (0, sqrt(3)), NW); label("$N$", (1, sqrt(3)), NE); [/asy]

Refer to the diagram above.

By symmetry, $A$ lies on the line $y = x$. The equation of the side of the top hexagon that $AB$ is on is $y = -x\sqrt{3} + \frac{3}{2}\sqrt{3} - \frac{1}{2}$. Thus, we can solve for the coordinates of $A$: \[x = -x\sqrt{3} + \frac{3\sqrt{3} - 1}{2}\] \[x(\sqrt{3} + 1) = \frac{3\sqrt{3} - 1}{2}\] \[x = \frac{3\sqrt{3}-1}{2} \cdot \frac{1}{\sqrt{3} + 1}\] \[= \frac{3\sqrt{3}-1}{2(\sqrt{3} + 1)} \cdot \frac{\sqrt{3} - 1}{\sqrt{3} - 1}\] \[= \frac{10 - 4\sqrt{3}}{4}\] \[= \frac{5}{2} - \sqrt{3}\] \[\therefore x, y = \frac{5}{2} - \sqrt{3}.\]

This means that we can find the length $AB$, which is equal to $2(\frac{5}{2} - \sqrt{3}) = (5 - 2\sqrt{3}$. We will next find the area of trapezoid $ABMN$. The lengths of the bases are $1$ and $5 - 2\sqrt{3}$, and the height is equal to the $y$-coordinate of $M$ minus the $y$-coordinate of $A$. The height of the hexagon is $\sqrt{3}$ and the bottom of the hexagon lies on the line $y = \frac{1}{2}$. Thus, the $y$-coordinate of $M$ is $\sqrt{3} - \frac{1}{2}$, and the height is $2\sqrt{3} - 3$. We can now find the area of the trapezoid: \[[ABMN] = (2\sqrt{3} - 3)\left(\frac{1 + 5 - 2\sqrt{3}}{2}\right)\] \[= (2\sqrt{3} - 3)(3 - \sqrt{3})\] \[= 6\sqrt{3} + 3\sqrt{3} - 9 - 6\] \[= 9\sqrt{3} - 15.\]

The total area of the figure is the area of a square with side length $AB$ plus four times the area of this trapezoid: \[A = (5 - 2\sqrt{3})^2 + 4(9\sqrt{3} - 15)\] \[= 37 - 20\sqrt{3} + 36\sqrt{3} - 60\] \[= 16\sqrt{3} - 23.\]

Our answer is $16 + 3 - 23 = \boxed{\textbf{(B) }-4}$.

~mathboy100

Solution 2

We calculate the area as the area of the red octagon minus the four purple congruent triangles: [asy] import geometry; unitsize(3cm); draw((1-sqrt(3),1-sqrt(3))--(1-sqrt(3),sqrt(3))--(sqrt(3),sqrt(3))--(sqrt(3),1-sqrt(3))--cycle,dashed); filldraw((0,1-sqrt(3))--(1,1-sqrt(3))--(sqrt(3),0)--(sqrt(3),1)--(1,sqrt(3))--(0,sqrt(3))--(1-sqrt(3),1)--(1-sqrt(3),0)--cycle,red*0.2+white,red); filldraw((1,1-sqrt(3))--(3-sqrt(3),sqrt(3)-2)--(sqrt(3),0)--cycle,purple*0.2+white,blue); filldraw((sqrt(3),1)--(3-sqrt(3),3-sqrt(3))--(1,sqrt(3))--cycle,purple*0.2+white,blue); filldraw((0,sqrt(3))--(sqrt(3)-2,3-sqrt(3))--(1-sqrt(3),1)--cycle,purple*0.2+white,blue); filldraw((0,1-sqrt(3))--(1-sqrt(3),0)--(sqrt(3)-2,sqrt(3)-2)--cycle,purple*0.2+white,blue); draw((0,0) -- (1,0) -- (1,1) -- (0,1) -- cycle); draw(shift((1/2,1-sqrt(3)/2))*polygon(6)); draw(shift((1/2,sqrt(3)/2))*polygon(6)); draw(shift((sqrt(3)/2,1/2))*rotate(90)*polygon(6)); draw(shift((1-sqrt(3)/2,1/2))*rotate(90)*polygon(6)); [/asy] We first find the important angles in the figure. We note that 2 adjacent hexagons are rotated $90^\circ$ with respect to the other, so the angles between any sides is $150^\circ$. In particular, as the purple triangles are isosceles, they have angles $150^\circ,15^\circ$, and $15^\circ$, and the octagon is equiangular (all its angles are $135^\circ$). Thus, we can draw a square around the octagon, and we note that the ``cut out" triangles are all isosceles right triangles.

Now, we calculate the side length of the square. Note that the hexagon has a height of $\sqrt 3$, so the length of a side of the square is $2\sqrt 3-1$. In particular, the horizontal/vertical sides of the octagon have length $1$, so the legs of the isosceles triangles are \[\frac{2\sqrt3-1-1}2=\sqrt3-1\]Thus, the area of the octagon is \[(2\sqrt3-1)^2-4\cdot\frac 12(\sqrt3-1)^2=5\]Now, we calculate the area of one of the four isosceles triangles. The base of the triangle is $(\sqrt 3-1)\sqrt 2$, so the area is \[\frac 14\mathrm{base length}^2\cdot\tan(\mathrm{base angle})=\frac 14((\sqrt3-1)\sqrt2)^2\cdot\tan15^\circ=\frac 14(8-4\sqrt3)(2-\sqrt3)=7-4\sqrt 3\]Thus, the area of the dodecagon is \[5-4(7-4\sqrt3)=16\sqrt3-23\]Thus the answer is $16+3-23=-4$, or $\boxed{\textbf{(B)}}$.

~cr. naman12

Video Solution

https://youtu.be/QYclqXWnxxE

~Steven Chen (Professor Chen Education Palace, www.professorchenedu.com)

See Also

2022 AMC 12B (Problems • Answer Key • Resources)
Preceded by
Problem 24 		Followed by
Last problem
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=2022_AMC_12B_Problems/Problem_25&oldid=182601"