Answer:
The random variables in this case are discrete since they have a Multinomial distribution.
The probability mass function for a discrete random variable X is given by:
[tex]P(X=x_{i} )[/tex]
Where are [tex]x_{i}[/tex] are possible values of X.
The joint probability mass function of two discrete random variables X and Y is defined as
P(x,y) =P(X=x,Y=y).
It follows that, The joint probability mass function of [tex]X_{3} , X_{4}[/tex] is :
[tex]P(X_{3}, X_{4} ) = P( X_{3} = x_{3}, X_{4} = x_{4} ) =\frac{1}{8} +\frac{3}{8} =\frac{1}{2}[/tex]
