Simple question about probability

In summary, the conversation discusses strategies for calculating the number of pairs (X,Y) that satisfy the condition X²+Y²≤N². While drawing the graph and using Mathematica can provide insight for large values of N, the general formula for calculating the number of pairs as a function of N is not known. The question does not have a strong connection to probability unless the random variables X and Y have a specific distribution.
  • #1
nezse
7
0
Hi All,
I'm stucked computing this:
I have two discrete random variables 1≤X≤N and 1≤Y≤N. How many pairs of (X,Y) satisfy X²+Y²≤N²

I began by using a certain value for N and trying to search for patterns in the numbers that satisfy this constraint but I can't seem to get any meaningful pattern.

Any ideas?
Thanks.
 
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  • #2
Try drawing it out. Draw the region 1<=X<=N and 1<=Y<=N in the X-Y plane. Then draw in the condition X^2 +Y^2 <= N^2. Does this help?
 
  • #3
At first I draw it in a paper with N=9. And that give me no clue. Today, with the purpose to show you the figure, I made it again, but in Mathematica. When I see the elementes that satisfy the condition in that matrix, the answer to N→∞ become so obvius. I attach the image.

When N→∞ the amount of numbers that satisty X2+Y2≤N2 is Pi*N2/4, because X,Y>0. If X and Y be any real number equal to the restriction is an equation of a circle, treats including its inside.
For small N, I still have no idea.

Thanks, phyzguy.
 
  • #4
I forgot to post the images.
 

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  • #5
nezse said:
At first I draw it in a paper with N=9. And that give me no clue. Today, with the purpose to show you the figure, I made it again, but in Mathematica. When I see the elementes that satisfy the condition in that matrix, the answer to N→∞ become so obvius. I attach the image.

When N→∞ the amount of numbers that satisty X2+Y2≤N2 is Pi*N2/4, because X,Y>0. If X and Y be any real number equal to the restriction is an equation of a circle, treats including its inside.
For small N, I still have no idea.

Thanks, phyzguy.

Since the title of the thread said it was a probability question, I assumed it was in the large N limit, since this is what a probability is. Are you supposed to calculate the number of pairs as a function of N. If so, I don't know how to write a general formula.
 
  • #6
If the question is simply "how many integer pairs (X,Y) are there such that X2+Y2 <= N2", then that doesn't have a lot to do with probability. Unless your random variables X and Y have some specific distribution besides the uniform distribution that you neglected to tell us, this seems to be the question you're asking.
 
  • #7
I agree, my fault. The thing is i need to calculate the probability of that event, so I need to know how many pairs satisfy that condition. I already know that the probability of any pair to come up is 1/N^2, therefore I only need to know how many pairs like that can come up and multiply that number by 1/N^2. That's why I asked for the number of pairs.
 

1. What is probability?

Probability is a measure of the likelihood or chance of an event occurring. It is often represented as a number between 0 and 1, where 0 indicates impossibility and 1 indicates certainty.

2. How is probability calculated?

Probability is calculated by dividing the number of favorable outcomes by the total number of possible outcomes. This is known as the classical probability formula.

3. What is the difference between theoretical and experimental probability?

Theoretical probability is based on the assumption that all outcomes are equally likely, while experimental probability is based on actual results from an experiment or observation.

4. Can probability be greater than 1?

No, probability cannot be greater than 1. A probability of 1 indicates certainty, and anything greater than 1 would mean that the event is more than certain, which is not possible.

5. How is probability used in real life?

Probability is used in various fields, such as statistics, mathematics, and science, to make predictions and decisions based on uncertain events. It is also used in everyday situations, such as predicting the weather or the likelihood of winning a game.

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