Nested Hexagons

Geometry Level 4

Extend the diagram above to an infinite number of inscribed hexagons.

Let H j H_{j} be the j t h j_{th} regular hexagon and H j + 1 H_{j + 1} be inscribed in H j H_{j} , where each vertex of H j + 1 H_{j + 1} touches each midpoint of each side of H j H_{j} .

For j 0 j \geq 0 let A j A_{j} be the area of j t h j_{th} hexagon.

If A = j = 0 A j A = \sum_{j = 0}^{\infty} A_{j} , find A A 0 \dfrac{A}{A_{0}} ..


The answer is 4.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

W L O G l e t S o = 1 b e t h e s i d e o f t h e f i r s t h e x a g o n . . A h e x a g o n i s f o r m e d b y s i x e q u i l a t e r a l Δ s , w i t h s i d e = S . r a t i o o f c o r r e s p o n d i n g e q u i l a t e r a l Δ s , w i l l b e t h e s a m e a s t h e o n e r e q u i r e d . A s s e e n f r o m t h e s k e t c h t h e s i d e r a t i o s a r e 3 2 . a r e a r a t i o s , A 1 / A o = ( 3 2 ) 2 = 3 4 . w e h a v e a r e a s u m a s : A = 3 4 { 1 2 + 3 4 + ( 3 4 ) 2 + ( 3 4 ) 3 . . . } = 3 4 n = 0 ( 3 4 ) n . T h i s i s a G . P . s o s u m a 1 r = 1 1 3 4 = 4. S o A A o = 3 4 4 3 4 1 = 4. WLOG~let~S_o=1~be~the~side~of~ the~first~hexagon..\\ A~hexagon~is ~formed~by~six~equilateral~\Delta s, ~with ~side = S.\\ \implies~ratio~of~corresponding~equilateral~\Delta s,~will~be~the~same~as~the~one~required.\\ As~seen~from~the~sketch~the~side~ratios~are~\dfrac {\sqrt3} 2.\\ \implies~area~ratios, A_1/A_o=\left ( \dfrac {\sqrt3} 2 \right )^2=\dfrac 3 4 .\\ \therefore ~we~have~area~sum~as~:~~~A = \dfrac{\sqrt3} 4* \left \{1^2+\frac 3 4+(\frac 3 4)^2+(\frac 3 4)^3 . . .\right \} \\ =\displaystyle~~\frac{\sqrt3} 4* \sum_{n=0}^{\infty} \left (\frac 3 4 \right )^n. \\ This~is~a~G.P.~so~sum~\dfrac a {1-r}=\dfrac 1{1-\frac 3 4}=4.\\ So~\dfrac A{ A_o} =\dfrac {\dfrac{\sqrt3} 4 *4}{\dfrac{\sqrt3} 4 *1}=\huge \color{#D61F06}{4}.\\

Rocco Dalto
Jul 27, 2018

Let x 0 x_{0} be a side of the initial hexagon \implies the height h 0 h_{0} of the initial hexagon is h 0 = 3 2 x 0 = x 1 h_{0} = \dfrac{\sqrt{3}}{2}x_{0} = x_{1}

h 1 = 3 2 h 0 = ( 3 2 ) 2 x 0 = x 2 h 2 = 3 2 h 1 = ( 3 2 ) 3 x 0 = x 3 \implies h_{1} = \dfrac{\sqrt{3}}{2}h_{0} = (\dfrac{\sqrt{3}}{2})^2 x_{0} = x_{2} \implies h_{2} = \dfrac{\sqrt{3}}{2}h_{1} = (\dfrac{\sqrt{3}}{2})^3 x_{0} = x_{3} and h j 1 = ( 3 2 ) j x 0 = x j h j = ( 3 2 ) j + 1 x 0 h_{j - 1} = (\dfrac{\sqrt{3}}{2})^{j} x_{0} = x_{j} \implies h_{j} = (\dfrac{\sqrt{3}}{2})^{j + 1} x_{0} .

x 0 3 2 x 0 3 2 x 0 ( 3 2 ) 2 x 0 ( 3 2 ) 2 x 0 ( 3 2 ) 3 x 0 . . . . . . ( 3 2 ) j x 0 ( 3 2 ) j + 1 x 0 \begin{vmatrix}{x_{0}} && {\dfrac{\sqrt{3}}{2}x_{0}} \\ {\dfrac{\sqrt{3}}{2}x_{0}} && {(\dfrac{\sqrt{3}}{2})^2 x_{0}} \\ {(\dfrac{\sqrt{3}}{2})^2 x_{0}} && {(\dfrac{\sqrt{3}}{2})^3 x_{0}} \\ {...} && {...} \\ {(\dfrac{\sqrt{3}}{2})^{j} x_{0}} && {(\dfrac{\sqrt{3}}{2})^{j + 1} x_{0}} \end{vmatrix}

A j = 6 ( 1 2 ) ( 3 2 ) j ( 3 2 ) j + 1 x 0 2 = 3 ( 3 2 ) 2 j + 1 x 0 2 = 3 3 2 x 0 2 ( 3 4 ) j = ( 3 4 ) j A 0 \implies A_{j} = 6(\dfrac{1}{2})(\dfrac{\sqrt{3}}{2})^{j}(\dfrac{\sqrt{3}}{2})^{j + 1} {x_{0}}^2 = 3(\dfrac{\sqrt{3}}{2})^{2j + 1} x_{0}^2 = \dfrac{3\sqrt{3}}{2} {x_{0}}^2 (\dfrac{3}{4})^{j} = (\dfrac{3}{4})^{j} A_{0} \implies

A = j = 0 A j = A 0 j = 0 ( 3 4 ) j = 4 A 0 A A 0 = 4 A = \sum_{j = 0}^{\infty} A_{j} = A_{0}\sum_{j = 0}^{\infty} (\dfrac{3}{4})^j = 4A_{0} \implies \dfrac{A}{A_{0}} = \boxed{4} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...