A fair coin is flipped twice. Let X be the number of heads in the two tosses, and Y be the indicator r.v. for the tosses landing the same way. That is, Y is equal to 1 if the tosses land in the same way, and 0 otherwise. (a) Find the joint pmf of X and Y.

Respuesta :

Answer:

Step-by-step explanation:

It can be clearly observed that [tex]X\in{0,1,2}[/tex]  and [tex]Y\in {0,1}[/tex]

Now lets find the probabilities of all possible combinations

[tex]P(X=0, Y=0)=P(Y=0|X=0)P(X=0)=0[/tex]

Since it is impossible that the coin ha flipped in different ways given that all the time the tail has fallen

[tex]P(X=1,Y=0)=P(Y=0|X=1)P(X=1)=1\times2\times(\frac{1}{2} )^2[/tex] =1/2

[tex]P(X=2,Y=0)=P(Y=0|X=2)P(X=2)=1\times2\times(\frac{1}{2} )^2[/tex] =1/2

and similarly,

[tex]P(X=0,Y=1)=P(Y=1|X=0)P(X=0)=1\times(\frac{1}{2} )^2= 1/4[/tex]

[tex]P(X=1,Y=1)=P(Y=1|X=1)P(X=1)=0[/tex]

[tex]P(X=2,Y=1)=P(Y=1|X=2)P(X=2)=1\times(\frac{1}{2} )^2=1/4[/tex]