9. Correct. The answer is true. To prove this proposition, it is necessary to calculate the marginal densities for x and y respectively and show that:f(x,y) = f_X(x).f_Y(y), where f_i(i) is the marginal density function: f_i(i) = ∫^b_a f(i,j)dj.

Then:

Therefore, (x,y) is a random vector with independent variables.