Wedged in equilateral triangles 2.

Geometry Level 3

In square A B C D ABCD , four congruent equilateral triangles are wedged in to form the inner red square.

Let O O be the center of the red square and point Q Q a vertex of the red square and fold O Q \overline{OQ} at a right angle, as shown above, to form the height of the right square pyramid.

Let V p V_{p} be the volume of the right square pyramid and V C V_{C} be the volume of the cube formed using square A B C D ABCD .

If V P V C = ( α β λ ) λ γ λ γ \dfrac{V_{P}}{V_{C}} = \dfrac{(\alpha - \beta\sqrt{\lambda})^{\dfrac{\lambda}{\gamma}}}{\lambda\sqrt{\gamma}} , where α , β . λ \alpha,\beta.\lambda and γ \gamma are coprime positive integers, find α + β + λ + γ \alpha + \beta + \lambda + \gamma .


The answer is 16.

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2 solutions

Rocco Dalto
Jan 3, 2021

Let m m be a side of square A B C D ABCD .

Let A e A_{e} be the area of equilateral D F E \triangle{DFE} 4 A e = 4 ( 1 2 ) ( a ) ( 3 2 a ) = 3 a 2 \implies 4A_{e} = 4(\dfrac{1}{2})(a)(\dfrac{\sqrt{3}}{2}a) = \sqrt{3}a^2

Let A I A_{I} be the area of G F D \triangle{GFD}

The height h G F D = ( m a ) sin ( 6 0 ) = 3 2 ( m a ) h_{\triangle{GFD}} = (m - a)\sin(60^{\circ}) = \dfrac{\sqrt{3}}{2}(m - a) \implies

4 A I = 4 ( 1 2 ) ( m a ) 2 ( 3 2 ) = 3 ( m a ) 2 4A_{I} = 4(\dfrac{1}{2})(m - a)^2(\dfrac{\sqrt{3}}{2}) = \sqrt{3}(m - a)^2

Using the law of cosines on G F D \triangle{GFD} with included F G D \angle{FGD} \implies

a 2 = 2 ( m a ) 2 ( 3 2 ) = 3 ( m a ) 2 = 3 a 2 6 m a + 3 m 2 2 a 2 6 m a + 3 m 2 = 0 a^2 = 2(m - a)^2 (\dfrac{3}{2}) = 3(m - a)^2 = 3a^2 -6ma + 3m^2 \implies 2a^2 - 6ma + 3m^2 = 0

a = 3 ± 3 2 m \implies a = \dfrac{3 \pm \sqrt{3}}{2}m

a = 3 + 3 2 m m a = ( 1 + 3 ) m < 0 a = \dfrac{3 + \sqrt{3}}{2}m \implies m - a = -(1 + \sqrt{3})m < 0

a = 3 3 2 m m a = 3 1 2 m a = \dfrac{3 - \sqrt{3}}{2}m \implies m - a = \dfrac{\sqrt{3} - 1}{2}m

\therefore we drop a = 3 + 3 2 m a = \dfrac{3 + \sqrt{3}}{2}m and choose a = 3 3 2 m a = \dfrac{3 - \sqrt{3}}{2}m

4 A e = 3 4 ( 3 3 ) 2 m 2 = 3 3 2 ( 2 3 ) m 2 \implies 4A_{e} = \dfrac{\sqrt{3}}{4}(3 - \sqrt{3})^2 m^2 = \dfrac{3\sqrt{3}}{2}(2 - \sqrt{3}) m^2

and

4 A I = 3 ( m a ) 2 = 3 4 ( 3 1 ) 2 m 2 = 3 2 ( 2 3 ) m 2 4A_{I} = \sqrt{3}(m - a)^2 = \dfrac{\sqrt{3}}{4}(\sqrt{3} - 1)^2 m^2 = \dfrac{\sqrt{3}}{2}(2 - \sqrt{3})m^2

Let A s A_{s} be the area of the red square

A s = m 2 ( 4 A e + 4 A I ) = ( 1 ( 2 3 ( 2 3 ) ) ) m 2 = \implies A_{s} = m^2 - (4A_{e} + 4A_{I}) = (1 - (2\sqrt{3}(2 - \sqrt{3})))m^2 = ( 1 ( 4 3 6 ) ) m 2 (1 - (4\sqrt{3} - 6))m^2

= ( 7 4 3 ) m 2 = (7 - 4\sqrt{3})m^2

Let y y be a side of the red square y = 7 4 3 m O Q = 2 2 7 4 3 m = 7 4 3 2 m \implies y= \sqrt{7 - 4\sqrt{3}}m \implies \overline{OQ} = \dfrac{\sqrt{2}}{2}\sqrt{7 - 4\sqrt{3}}m = \sqrt{\dfrac{7 - 4\sqrt{3}}{2}}m \implies

V p = 1 3 A s O Q = ( 7 4 3 ) 3 2 3 2 m 3 V_{p} = \dfrac{1}{3}A_{s}\overline{OQ} = \dfrac{(7 - 4\sqrt{3})^{\dfrac{3}{2}}}{3\sqrt{2}}m^3 \implies

V P V C = ( 7 4 3 ) 3 2 3 2 = ( α β λ ) λ γ λ γ \dfrac{V_{P}}{V_{C}} = \dfrac{(7 - 4\sqrt{3})^{\dfrac{3}{2}}}{3\sqrt{2}} = \dfrac{(\alpha - \beta\sqrt{\lambda})^{\dfrac{\lambda}{\gamma}}}{\lambda\sqrt{\gamma}} \implies

α + β + λ + γ = 16 \alpha + \beta + \lambda + \gamma = \boxed{16} .

For simplicity let A B C D ABCD be a unit square. Then, the volume of the cube is V C = 1 {{V}_{C}}=1 .

Now, with the labelling of the figure where a a is the side length of the equilateral triangles and s s is the side length of the inner red square, we focus on F E B \triangle FEB :
F E B = 120 \angle FEB= 120{}^\circ and F B E = 30 \angle FBE= 30{}^\circ , hence E F B = 30 \angle EFB= 30{}^\circ , i..e. F E B \triangle FEB is isosceles, with E B = E F = 1 a EB=EF=1-a .

By sine rule, F B sin E = F E sin B a sin 120 = 1 a sin 30 1 2 a = 3 2 ( 1 a ) a = 3 3 2 \dfrac{FB}{\sin E}=\dfrac{FE}{\sin B}\Rightarrow \dfrac{a}{\sin 120{}^\circ }=\dfrac{1-a}{\sin 30{}^\circ }\Rightarrow \dfrac{1}{2}a=\dfrac{\sqrt{3}}{2}\left( 1-a \right)\Rightarrow a=\dfrac{3-\sqrt{3}}{2} Furthermore, s = G F = G E F E = a ( 1 a ) = 2 a 1 = 2 3 3 2 1 s = 2 3 \begin{aligned} & s=GF=GE-FE=a-\left( 1-a \right)=2a-1=2\dfrac{3-\sqrt{3}}{2}-1 \\ & \Rightarrow s=2-\sqrt{3} \\ \end{aligned} For the height h h of the pyramid, h = O Q = G Q 2 = s 2 2 = s 2 h=OQ=\dfrac{GQ}{2}=\dfrac{s\sqrt{2}}{2}=\dfrac{s}{\sqrt{2}} Thus, the volume of the pyramid is V P = 1 3 s 2 h = 1 3 s 3 2 = ( 2 3 ) 3 3 2 = [ ( 2 3 ) 2 ] 3 2 3 2 = ( 7 4 3 ) 3 2 3 2 {{V}_{P}}=\dfrac{1}{3}{{s}^{2}}h=\dfrac{1}{3}\cdot \dfrac{{{s}^{3}}}{\sqrt{2}}=\dfrac{{{\left( 2-\sqrt{3} \right)}^{3}}}{3\sqrt{2}}=\dfrac{{{\left[ {{\left( 2-\sqrt{3} \right)}^{2}} \right]}^{\dfrac{3}{2}}}}{3\sqrt{2}}=\dfrac{{{\left( 7-4\sqrt{3} \right)}^{\dfrac{3}{2}}}}{3\sqrt{2}} V P V C = V P 1 = ( 7 4 3 ) 3 2 3 2 \Rightarrow \dfrac{{{V}_{P}}}{{{V}_{C}}}=\dfrac{{{V}_{P}}}{1}=\dfrac{{{\left( 7-4\sqrt{3} \right)}^{\dfrac{3}{2}}}}{3\sqrt{2}} Hence, α = 7 \alpha =7 , β = 4 \beta =4 , λ = 3 \lambda =3 , γ = 2 \gamma =2 and α + β + λ + γ = 7 + 4 + 3 + 2 = 16 \alpha+\beta+\lambda+\gamma =7+4+3+2=\boxed{16} .

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