Difference between revisions of "2021 Fall AMC 12A Problems/Problem 14"
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<math>\textbf{(A)} \: 4 \qquad \textbf{(B)} \: 4\sqrt3 \qquad \textbf{(C)} \: 12 \qquad \textbf{(D)} \: 18 \qquad \textbf{(E)} \: 12\sqrt3</math> | <math>\textbf{(A)} \: 4 \qquad \textbf{(B)} \: 4\sqrt3 \qquad \textbf{(C)} \: 12 \qquad \textbf{(D)} \: 18 \qquad \textbf{(E)} \: 12\sqrt3</math> | ||
− | ==Solution (Law of Cosines and Equilateral Triangle Area)== | + | ==Solution 1 (Law of Cosines and Equilateral Triangle Area)== |
Isosceles triangles <math>ABF</math>, <math>CBD</math>, and <math>EDF</math> are congruent by [[Congruent_(geometry)#SAS_Congruence|SAS congruence]]. By [[CPCTC]], <math>BF=BD=DF</math>, so triangle <math>BDF</math> is equilateral. | Isosceles triangles <math>ABF</math>, <math>CBD</math>, and <math>EDF</math> are congruent by [[Congruent_(geometry)#SAS_Congruence|SAS congruence]]. By [[CPCTC]], <math>BF=BD=DF</math>, so triangle <math>BDF</math> is equilateral. | ||
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The total area of the hexagon is thrice the area of each isosceles triangle plus the area of the equilateral triangle, or <math>3\left(\frac{1}{4}s^2\right)+\frac{\sqrt{3}}{2}s^2-\frac{3}{4}s^2=\frac{\sqrt{3}}{2}s^2=6\sqrt{3}</math>. Hence, <math>s=2\sqrt{3}</math> and the perimeter is <math>6s=\boxed{\textbf{(E)} \: 12\sqrt{3}}</math>. | The total area of the hexagon is thrice the area of each isosceles triangle plus the area of the equilateral triangle, or <math>3\left(\frac{1}{4}s^2\right)+\frac{\sqrt{3}}{2}s^2-\frac{3}{4}s^2=\frac{\sqrt{3}}{2}s^2=6\sqrt{3}</math>. Hence, <math>s=2\sqrt{3}</math> and the perimeter is <math>6s=\boxed{\textbf{(E)} \: 12\sqrt{3}}</math>. | ||
+ | |||
+ | == Solution 2 == | ||
+ | We denote by <math>x</math> the side length. | ||
+ | |||
+ | Following from SAS, <math>\triangle ABF \cong \triangle CDB \cong \triangle EFD</math>. | ||
+ | |||
+ | Hence, <math>BF =BD = DF</math>. Hence, <math>\triangle BDF</math> is equilateral. | ||
+ | |||
+ | In <math>\triangle ABF</math>, by applying the law of cosines, we have | ||
+ | <cmath> | ||
+ | \begin{align*} | ||
+ | BF^2 & = AB^2 + AF^2 - 2 AB \cdot AF \cdot \cos A \\ | ||
+ | & = x^2 + x^2 - 2x^2 \cos 30^\circ \\ | ||
+ | & = x^2 \left( 2 - \sqrt{3} \right) . | ||
+ | \end{align*} | ||
+ | </cmath> | ||
+ | |||
+ | Therefore, | ||
+ | <cmath> | ||
+ | \begin{align*} | ||
+ | {\rm Area} \ ABCDEF | ||
+ | & = {\rm Area} \ \triangle ABF + {\rm Area} \ \triangle CDB + {\rm Area} \ \triangle EFD | ||
+ | + {\rm Area} \ \triangle BDF \\ | ||
+ | & = 3 {\rm Area} \ \triangle ABF + {\rm Area} \ \triangle BDF \\ | ||
+ | & = 3 \cdot \frac{1}{2} AB \cdot AF \cdot \sin A | ||
+ | + \frac{\sqrt{3}}{4} BF^2 \\ | ||
+ | & = 3 \cdot \frac{1}{2} x^2 \sin 30^\circ | ||
+ | + \frac{\sqrt{3}}{4} x^2 \left( 2 - \sqrt{3} \right) \\ | ||
+ | & = \frac{\sqrt{3}}{2} x^2 . | ||
+ | \end{align*} | ||
+ | </cmath> | ||
+ | |||
+ | Because <math>{\rm Area} \ ABCDEF = 6 \sqrt{3}</math>, <math>x = 2 \sqrt{3}</math>. | ||
+ | Therefore, the perimeter of <math>ABCDEF</math> is <math>6 x = 12 \sqrt{3}</math>. | ||
+ | |||
+ | Therefore, the answer is <math>\boxed{\textbf{(E) }12 \sqrt{3}}</math>. | ||
+ | |||
+ | ~Steven Chen (www.professorchenedu.com) | ||
+ | |||
{{AMC12 box|year=2021 Fall|ab=A|num-b=13|num-a=15}} | {{AMC12 box|year=2021 Fall|ab=A|num-b=13|num-a=15}} | ||
{{MAA Notice}} | {{MAA Notice}} |
Revision as of 20:46, 25 November 2021
Problem
In the figure, equilateral hexagon has three nonadjacent acute interior angles that each measure . The enclosed area of the hexagon is . What is the perimeter of the hexagon?
Solution 1 (Law of Cosines and Equilateral Triangle Area)
Isosceles triangles , , and are congruent by SAS congruence. By CPCTC, , so triangle is equilateral.
Let the side length of the hexagon be .
The area of each isosceles triangle is by the fourth formula here.
By the Law of Cosines on triangle , . Hence, the area of the equilateral triangle is .
The total area of the hexagon is thrice the area of each isosceles triangle plus the area of the equilateral triangle, or . Hence, and the perimeter is .
Solution 2
We denote by the side length.
Following from SAS, .
Hence, . Hence, is equilateral.
In , by applying the law of cosines, we have
Therefore,
Because , . Therefore, the perimeter of is .
Therefore, the answer is .
~Steven Chen (www.professorchenedu.com)
2021 Fall AMC 12A (Problems • Answer Key • Resources) | |
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Followed by Problem 15 |
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