Stacking Squares in a Triangle!

Geometry Level 2

If we continue to stack squares inside an equilateral triangle of side length 1 1 . What is the total area of the squares?

Note : I upgraded an old problem of my posts by adding some animation.

3 3 3 3\sqrt{3} - 3 1 2 ( 3 3 3 ) \frac{1}{2} (3\sqrt{3} -3) 3 3 1 4 ( 3 3 ) \frac{1}{4}(3\sqrt{3}) 1 8 ( 3 3 3 ) \frac{1}{8}(3\sqrt{3} - 3)

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4 solutions

David Vreken
May 18, 2020

Consider one trapezoid layer, where the short leg of one of the side triangles is x x , which makes the side of the square 3 x \sqrt{3}x .

Then the ratio of the area of the square to the area of the trapezoid is ( 3 x ) 2 ( 3 x ) 2 + 3 x 2 = 1 2 ( 3 3 ) \frac{(\sqrt{3}x)^2}{(\sqrt{3}x)^2 + \sqrt{3}x^2} = \frac{1}{2}(3 - \sqrt{3}) .

Since this ratio is maintained for the infinite layers of trapezoids in the diagram, and since the area of the whole equilateral triangle is 3 4 1 2 = 3 4 \frac{\sqrt{3}}{4} \cdot 1^2 = \frac{\sqrt{3}}{4} , the sum of the areas of the stacked squares is 1 2 ( 3 3 ) 3 4 = 1 8 ( 3 3 3 ) \frac{1}{2}(3 - \sqrt{3}) \cdot \frac{\sqrt{3}}{4} = \boxed{\frac{1}{8}(3\sqrt{3} - 3)} .

Thank you.

Hana Wehbi - 1 year ago
Chew-Seong Cheong
May 19, 2020

Order the square from the large to small. Let the first square to have a side length of a a . Then the ratio of side length of first square and the base of the triangle is a 1 \dfrac a1 . Because the pattern repeats, the ratio of the side length of the second side length and the base of second triangle is also a 1 = a 2 a \dfrac a1 = \dfrac {a^2}a . Therefore the side length of the second square is a 2 a^2 . That of the third is a 3 a^3 and so on. The the sum of areas of all the square is given by:

S A = a 2 + a 4 + a 6 + = a 2 1 a 2 From 2 a tan 3 0 + a = 1 , we have = 3 ( 7 4 3 ) 4 ( 3 3 5 ) = 3 ( 3 1 ) 8 a = 3 ( 2 3 ) , a 2 = 3 ( 7 4 3 ) \begin{aligned} S_A & = a^2 + a^4 + a^6 + \cdots = \frac {a^2}{1-a^2} & \small \blue{\text{From }2 a \tan 30^\circ + a = 1 \text{, we have}} \\ & = \frac {3(7-4\sqrt 3)}{4(3\sqrt 3-5)} = \boxed{\frac {3(\sqrt 3-1)}8} & \small \blue{a = \sqrt 3(2-\sqrt 3), a^2 = 3(7-4\sqrt 3)} \end{aligned}

Thank you.

Hana Wehbi - 1 year ago

I think you have a typo. Shouldn’t a 3 a^3 be replaced with a 3 2 = a 6 a^{3^2} = a^6

Mahdi Raza - 1 year ago

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Thanks. It is a 3 × 2 a^{3\times 2} .

Chew-Seong Cheong - 1 year ago

Yes of course. Here it is :

Area of the largest square is

( 3 3 + 2 ) 2 (\frac{\sqrt 3}{\sqrt 3+2})^2 ,

the next smaller one is

( 3 3 + 2 ) 4 (\frac{\sqrt 3}{\sqrt 3+2})^4 ,

and so on.

Hence the total area of all the squares is

( 3 3 + 2 ) 2 × 1 1 ( 3 3 + 2 ) 2 = (\frac{\sqrt 3}{\sqrt 3+2})^2\times \dfrac{1}{1-(\frac{\sqrt 3}{\sqrt 3+2})^2}=

3 4 ( 3 + 1 ) = 1 8 ( 3 3 3 ) \dfrac{3}{4(\sqrt 3+1)}=\boxed {\dfrac{1}{8}(3\sqrt 3-3)} .

Thank you .

Hana Wehbi - 1 year ago
Hana Wehbi
May 18, 2020

Thank you everyone for sharing your solutions.

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