# Homework Help: Variables and their common density

1. May 11, 2013

### ParisSpart

we have X,Y variables and their common density f(m,n)=P(X=m,Y=n) where f(0,1)=0.1 f(1,0)=0.1
and f(1,1)=0,31 find P(X=0)

i think that P(X=0)= f(0,1) but it says that its incorrect what i am doing wrong?

2. May 11, 2013

### Staff: Mentor

I think you need more data.
Do you know that X and Y are independent? (my guess)
Do you know something else about X and Y?

Why do you think that? Does Y have to be 1 if X is 0?

3. May 11, 2013

### ParisSpart

the exercise gives only this data... its say that X,Y ,they take 0 and 1 each of them..

4. May 11, 2013

### Ray Vickson

You can determine f(0,0) by using the fact that the f(i,j) must sum to 1. What does the event {X=0} look like as a subset of the whole allowed (X,Y) space S= {(0,0),(0,1),(1,0),(1,1)}?

5. May 11, 2013

### ParisSpart

i must estimate f(0,0)?

6. May 11, 2013

### Ray Vickson

What do YOU think?

7. May 12, 2013

### ParisSpart

i dont understand how to solve this

8. May 12, 2013

### Ray Vickson

That must mean that you did not read my first response.

9. May 12, 2013

### ParisSpart

i read it but i dont understand very well english....may can u explain it more specifical

10. May 12, 2013

### haruspex

There are four combinations possible: 00, 01, 10, 11. You know the probabilities of three of these. So what is the probability of the fourth?

11. May 12, 2013

### ParisSpart

yea but what is the diference between 0.1 and 0,1 because he gives me decimal point and not 0,1

12. May 12, 2013

### Staff: Mentor

0.1 is a number, (0,1) are two numbers, here used for X and Y.