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Which one of the following shapes can be used to tile (completely cover by repeating) a flat plane, extending to infinity in all directions, without leaving any empty spaces in between them? The copies of the shape used to tile are identical and are not allowed to overlap.

  1. circle 
  2. regular octagon 
  3. regular pentagon 
  4. rhombus 

2 Answers

3 3 votes

Rhombus will be answer because it plays similar role as square .

Here  rectangle also work .

Rhombus can tile (completely cover by repeating) a flat plane, extending to infinity in all directions, without leaving any empty spaces in between them . Remaining options will always some empty spaces between them .

Correct Answer :- Option D) Rhombus

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Although the mathematics behind it comes from a very interesting field called "tessellation or tiling" but I can provide some intuition from my little understading.

1. When we are given a task of tiling then if we can somehow arrange that shape around a point such that the shapes cummulatively complete $360^{\circ}$ with their internal angles then that shape can fill the plane completely without leaving any empty space.

2. Let's understand this with the example of Rhombus:

A rhombus has the following properties:

  1. Opposite angles are always equal.
  2. $\alpha + \beta = 180^{\circ}$ where, $\alpha$ and $\beta$ are adjacent angles of rhombus.

So, if we form a group of rhombuses around a point such that there is always an angle $\alpha$ if there is an angle $\beta$ in the group and vice versa then they will cummulatively make $180^{\circ}$ and after $2$ such pairs we will have $360^{\circ}$ and that means there will be no empty space left on the plane. (fig. 1 and 2 in img 1 above)

But there is a catch, if we have a rhombus which has angles $\alpha$ and $\beta$ as factors of $360^{\circ}$ then we can take only one type of angle and achive the same results. (fig. 3 and 4 in img 1 above)

for example let's assume $\alpha = 60^{\circ}$ and $\beta = 120^{\circ}$ then $6 \times 60^{\circ} = 360^{\circ}$ and $3 \times 120^{\circ} = 360^{\circ}$.

3. Now, let's see why other shapes can't make tiling patterns:

a) Circles's story is crystal clear from the fig. 1 in img 2 above itself. There is no way we can remove the somewhat triangular looking space in center whenever $3$ circles meet around a point.

b) Regular pentagon has internal angle $= 108^{\circ}$ and if we try to find number of pentagons that we need around a point so that no empty space is left:

$n \times 108^{\circ} = 360^{\circ}$

$n = \frac{360^{\circ}}{108^{\circ}}$

$n \approx 3.33$

So, if we take $n = 3$ means only $3$ pentagons around a point (like fig. 2 in img 2 above) then $3 \times 108^{\circ} = 324^{\circ}$, which means there will be empty space left and if we take $n = 4$ means $4$ pentagons around a point (like fig. 3 in img 2 above) then $4 \times 108^{\circ} = 432^{\circ}$, which means there will be overlap b/w them.

c) Regular octagon has internal angle $= 135^{\circ}$ and if we try to find number of pentagons that we need around a point so that no empty space is left:

$n \times 135^{\circ} = 360^{\circ}$

$n = \frac{360^{\circ}}{135^{\circ}}$

$n \approx 2.66$

So, if we take $n = 2$ means only $2$ octagons around a point (like fig. 4 in img 2 above) then $2 \times 135^{\circ} = 270^{\circ}$, which means there will be empty space left and if we take $n = 3$ means $3$ octagons around a point (like fig. 5 in img 2 above) then $3 \times 135^{\circ} = 405^{\circ}$, which means there will be overlap b/w them.

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