Can 101 Discs of Radius 1/2 Cover a Rectangle That 25 Discs of Radius 1 Can?

In summary, the conversation discusses a question about covering a rectangle with discs of different radii. One person believes the question is incorrect and provides an example to support their argument. Another person helps solve the question by breaking the rectangle into smaller rectangles. The final conclusion is that the original rectangle can be covered with 100 discs of radius 1/2, with the 101st disc being redundant.
  • #1
caffeinemachine
Gold Member
MHB
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Hello MHB.
I am having trouble with the following quesion.

Let $R=\{(x,y)\in\mathbb R^2:A\leq x\leq B, C\leq y\leq D\}$ be a rectangle in $\mathbb R^2$ which can be covered (overlapping allowed) with $25$ discs of radius $1$ each.
Then $R$ can be covered with $101$ rectangles of radius $1/2$ each.

I think the question is wrong. I think that a rectangle having one side $1.01$ units and the other side just long enough so that $25$ discs of radius $1$ units can cover it will not be coverable by 101 discs of radius $1/2$.
I don't know how to prove this.

Can anybody help?
 
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  • #2
caffeinemachine said:
Hello MHB.
I am having trouble with the following quesion.

Let $R=\{(x,y)\in\mathbb R^2:A\leq x\leq B, C\leq y\leq D\}$ be a rectangle in $\mathbb R^2$ which can be covered (overlapping allowed) with $25$ discs of radius $1$ each.
Then $R$ can be covered with $101$ rectangles of radius $1/2$ each.

I think the question is wrong. I think that a rectangle having one side $1.01$ units and the other side just long enough so that $25$ discs of radius $1$ units can cover it will not be coverable by 101 discs of radius $1/2$.
I don't know how to prove this.

Can anybody help?

I discussed this with one of my friends and he solved it.

We note that any $a\times b$ rectangle is a union of 4 rectangles of dimension $(a/2)\times (b/2)$.

$R$ is a $A\times B$ rectabgle.
Say $R'$ denotes a copy of $(A/2)\times (B/2)$ rectangle.
Since $R$ can be covered by $25$ discs of unit radius, $R'$ can be covered by $25$ discs of radius $1/2$. Using $4$ different $R'$s we can cover $R$. Thus $R$ can be covered by a $100$ discs fo radius $1/2$. The $101$th disc is just for fun.
 

FAQ: Can 101 Discs of Radius 1/2 Cover a Rectangle That 25 Discs of Radius 1 Can?

How is it possible to cover a rectangle with 25 discs using only 101 discs of half the radius?

This is possible because the smaller discs can fit more tightly together than the larger ones, allowing for more coverage with fewer discs.

What is the significance of using a radius of 1/2 for the smaller discs?

A radius of 1/2 allows for a better fit and coverage of the rectangle, as it is exactly half the size of the larger discs and can fit more precisely into the gaps between them.

Can this concept be applied to other shapes besides a rectangle?

Yes, this concept can be applied to any shape that can be divided into smaller, equal parts. It is based on the principles of geometric packing and can be used to optimize space usage in various applications.

How does this concept relate to real-world applications?

This concept has many real-world applications, such as in packaging and storage, where optimizing space usage is important. It is also used in the design of computer chips and other technological devices to maximize efficiency.

Is there a limit to how many smaller discs can be used to cover the rectangle?

There is no specific limit, but as the number of smaller discs used increases, the coverage becomes more precise and efficient. In theory, an infinite number of smaller discs could be used to cover the rectangle perfectly.

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