Big Hexagon Small Hexagon

Geometry Level 3

The vertices of the purple hexagon are the midpoints of the sides of the green hexagon. Find the ratio of the area of the small hexagon (purple) to the area of the big hexagon (green).

2 3 \dfrac{2}{3} 3 4 \dfrac{3}{4} 4 5 \dfrac{4}{5} 5 6 \dfrac{5}{6}

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5 solutions

Michael Huang
Nov 15, 2020

As there are 3 × 6 = 18 3 \times 6 = 18 small triangles within the purple hexagon, there are 6 6 more triangles outside. Therefore, the area ratio is 18 18 + 6 = 3 4 \dfrac{18}{18+6} = \boxed{\dfrac{3}{4}} .

Chew-Seong Cheong
Nov 15, 2020

Let the radius of the big hexagon be 1 1 . Then the radius of the small hexagon r s = cos 3 0 = 3 2 r_s = \cos 30^\circ = \dfrac {\sqrt 3}2 and the ratio of their areas A s A B = r s 2 1 2 = ( 3 2 ) 2 = 3 4 \dfrac {A_s}{A_B} = \dfrac {r_s^2}{1^2}= \left(\dfrac {\sqrt 3}2 \right)^2 = \boxed {\dfrac 34} .

sin 60 = root3/2 is the ratio of the smaller to bigger side, square it and is the ratio of the area

Ron Gallagher
Nov 16, 2020

The area of a regular hexagon is proportional to the square of the length of a side (with a constant of proportionality of 3sqrt(3)/2, which is not relevant). Therefore, the ratio of the areas is the square of the ratio of the lengths of their sides. Without loss of generality, let the length of the side of the big hexagon be 2. Since each vertex of the small hexagon lies on the midpoint of a side of the big hexagon, and each internal angle of a hexagon is 120 degrees, if L is the side length of the small hexagon the Law of Cosines yields:

L^2 = 1^2 +2^2 - 2 (1) (1)*cos(120 degrees), or

L = sqrt(3).

Therefore, the ratio of the areas is:

(sqrt(3)/2)^2 = 3/4

N. Aadhaar Murty
Nov 17, 2020

Since, all regular polygons are cyclic and each side subtends an angle of 6 0 60^{\circ} at the center, we have 6 6 equilateral triangles as shown.

area = 6 3 4 s 2 \therefore \text{area} = 6 \cdot \frac {\sqrt {3}}{4}s^2

Also, since S 3 2 = s , \frac {S\sqrt {3}}{2} = s,

ratio of areas = s 2 S 2 = 3 4 \text {ratio of areas} = \frac {s^2}{S^2} = \boxed {\frac {3}{4}}

The expression for area can be neglected.

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