**13. Correct. The answer is true.** To obtain this model it is
necessary to find the conditional density function, which is defined as
follows:

First of all, we have to obtain the
marginal density *f _{1}*. It is
obtained by fixing

_{}

for all other values of y, *f _{1}(x)* = 0.

The conditional density function
of *x* is, for 0 < y < 1: