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Geometry Difficulty 3.9 AMC 10/12 Find the answer

Points K,L,M,K, L, M, and NN lie in the plane of the square ABCDABCD such that AKBAKB, BLCBLC, CMDCMD, and DNADNA are equilateral triangles. If ABCDABCD has an area of 16, find the area of KLMNKLMN.

(A) 32\textrm{(A)}\ 32(B) 16+163\textrm{(B)}\ 16+16\sqrt{3}(C) 48\textrm{(C)}\ 48(D) 32+163\textrm{(D)}\ 32+16\sqrt{3}(E) 64\textrm{(E)}\ 64

Multiple choice: answer with the letter of the option you want.

Solution

Solution 1
Since the area of square ABCD\text{ABCD} is 16, the side length must be 4. Thus, the side length of triangle AKB is 4, and the height of AKB\text{AKB}, and thus DMC\text{DMC}, is 232\sqrt{3}.
The diagonal of the square KNML\text{KNML} will then be 4+434+4\sqrt{3}. From here there are 2 ways to proceed:
First: Since the diagonal is 4+434+4\sqrt{3}, the side length is 4+432\frac{4+4\sqrt{3}}{\sqrt{2}}, and the area is thus 16+48+3232=(D) 32+163\frac{16+48+32\sqrt{3}}{2}=\boxed{\textbf{(D) } 32+16\sqrt{3}}.
Second: Since a square is a rhombus, the area of the square is d1d22\frac{d_1d_2}{2}, where d1d_1 and d2d_2 are the diagonals of the rhombus. Since the diagonal is 4+434+4\sqrt{3}, the area is (4+43)22=(D) 32+163\frac{(4+4\sqrt{3})^2}{2}=\boxed{\textbf{(D) } 32+16\sqrt{3}}.

Solution 2
Because ABCDABCD has area 1616, its side length is simply 16    4\sqrt{16}\implies 4.
Angle chasing, we find that the angle of KBL=360(90+2(60))=360(210)=150KBL=360-(90+2(60))=360-(210)=150.
We also know that KB=4,BL=4KB=4, BL=4.
Using Law of Cosines, we find that side (KL)2=16+162(16)cos150    (KL)2=32+3232    (KL)2=32+163(KL)^2=16+16-2(16)cos{150}\implies (KL)^2=32+\frac{32\sqrt{3}}{2}\implies (KL)^2=32+16\sqrt{3}.
However, the area of KLMNKLMN is simply (KL)2(KL)^2, hence the answer is (D) 32+163\boxed{\textbf{(D) } 32+16\sqrt{3}}

Solution 3
First we show that KLMNKLMN is a square. How? Show that it is a rhombus that has a right angle.
NAK=KBL=LCM=DNM=150\angle{NAK}=\angle{KBL}=\angle{LCM}=\angle{DNM}=150^\circ. All the sides of the equilateral triangles are equal, so the triangles are congruent. Notice that KNL=45\angle{KNL}=45^\circ, etc, so KNM=90\angle{KNM}=90^\circ. So we have a square.
Instead of going to find KNKN using Law of Cosines, we can inscribe the square in another bigger one and then subtract the four right triangles in the corners, so after all of this, we find that the answer is (D) 32+163\boxed{\textbf{(D) } 32+16\sqrt{3}}
~hastapasta (edited)

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.