Big Hexagon Small Hexagon - 2

Geometry Level pending

Vertices of the small hexagon (purple) divide the sides of the big hexagon (green) by a ratio of 1 : 2 1:2 in the same order.

Find the ratio of the area of the small hexagon to the area of the big hexagon.

7 8 \dfrac{7}{8} 7 9 \dfrac{7}{9} 7 10 \dfrac{7}{10} 4 5 \dfrac{4}{5}

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4 solutions

Michael Huang
Nov 15, 2020

Since the ratio values are integers, we can formulate the triangular lattices as shown above. From there, we see that there are 6 × 2 = 12 6 \times 2 = 12 triangles unshaded, whereas there are 4 × 6 2 + 6 + 12 × 2 = 42 4 \times \dfrac{6}{2} + 6 + 12 \times 2 = 42 triangles shaded. Thus, the area ratio is 42 42 + 12 = 7 9 \dfrac{42}{42 + 12} = \boxed{\dfrac{7}{9}} .

Ha, that's a genius idea!

Chris Lewis - 6 months, 3 weeks ago
Fletcher Mattox
Nov 15, 2020

Let A B = 1 AB = 1 , then G B = 2 3 , B H = 1 3 GB = \frac{2}{3},\quad BH = \frac{1}{3}

Then by the cosine rule, G H 2 = G B 2 + B H 2 2 G B B H cos 12 0 GH^2 = GB^2 + BH^2 - 2\cdot{GB}\cdot{BH}\cdot\cos{120^\circ}

G H 2 = ( 2 3 ) 2 + ( 1 3 ) 2 2 2 3 1 3 ( 1 2 ) = 7 9 GH^2 = (\frac{2}{3})^2 + (\frac{1}{3})^2 - 2\cdot\frac{2}{3}\cdot\frac{1}{3}\cdot(-\frac{1}{2}) = \frac{7}{9}

Since area is proportional to the square if the side, the solution is 7 9 \boxed{\frac{7}{9}} .

Chew-Seong Cheong
Nov 15, 2020

Note that the two hexagons share a center. Let the radius or side length of the big hexagon be 1 1 . Then the apothem of the big hexagon a B = 3 2 a_B = \dfrac {\sqrt 3}2 and the radius of the small hexagon is given by r s 2 = ( 3 2 ) 2 + ( 1 2 1 3 ) 2 = ( 3 2 ) 2 + ( 1 6 ) 2 = 7 9 r_s^2 = \left(\dfrac {\sqrt 3}2 \right)^2 + \left(\dfrac 12-\dfrac 13 \right)^2 = \left(\dfrac {\sqrt 3}2 \right)^2 + \left(\dfrac 16 \right)^2 = \dfrac 79 . And the ratio of areas A s A B = r s 2 1 2 = 7 9 \dfrac {A_s}{A_B} = \dfrac {r_s^2}{1^2} = \boxed{\frac 79} .

Hongqi Wang
Nov 14, 2020

a p u r p l e = ( 3 2 a g r e e n ) 2 + ( 1 6 a g r e e n ) 2 = 7 3 a g r e e n a_{purple} = \sqrt {(\frac {\sqrt 3}{2}a_{green})^2 + (\frac {1}{6}a_{green})^2} \\ = \frac {\sqrt 7}{3} a_{green}

S p u r p l e : S g r e e n = a p u r p l e 2 : a g r e e n 2 = 7 : 9 S_{purple} : S_{green} = a_{purple}^2 : a_{green}^2 = 7:9

Where do the 3 2 \dfrac{\sqrt{3}}{2} and 1 6 \dfrac{1}{6} come from ? I did not understand your solution.

Hosam Hajjir - 6 months, 4 weeks ago

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draw lines from center to two adjacent vertexs, then got an equilateral triangle, its attitude is 3 2 a \frac {\sqrt 3}2 a . the distance from vertex inner hexagon to mid of side of outer hexagon is 1 6 a \frac 16a .

Hongqi Wang - 6 months, 4 weeks ago

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