Reading and Problem Assignment #7
Math 501–1, Spring 2006
University of Utah
Read the section on “The distribution of a function of a random variable” in Chapter
5. (Section 5.7 in edition 7). Also start reading sections 6.1-6.2 of Chapter 6 (jointly
distributed random variables; edition 7).
The following are borrowed from your text.
Problems:
1. Suppose Yis uniformly distributed on (0 ,5). What is the probability that the roots
of the equation 4x2+ 4xY +Y+ 2 = 0 are both real?
2. Two fair dice are rolled. Find the joint mass function of (X , Y ) when:
(a) Xis the maximum (i.e., largest) of the values of the two dice, and Yis the sum
of the values of the two dice;
(b) Xis the value of the first die and Yis the maximum of the values of the two dice;
(c) Xis the minimum (i.e., smallest) of the values of the two dice, and Yis the
maximum of the two values.
3. Consider a sequence of independent Bernoulli trials, each of which is a success with
probability p. Let X1denote the number of failures preceding the first success, and
let X2be the number of failures between the first two successes. Find the joint mass
function of (X1, X2).
4. The joint density function of (X , Y ) is given by
f(x , y) = c(y2−x2)e−y,if −y≤x≤yand 0 < y < ∞,
0,otherwise.
(a) Find c.
(b) Find the (marginal) density functions of Xand Yrespectively.
(c) Find E(X).
(d) Find P{X > Y }.
5. The (joint) density function of (X , Y ) is given by
f(x , y) = e−(x+y),if 0 ≤x < ∞, and 0 ≤y < ∞,
0,otherwise.
Find: (a) P{X < Y }; and (b) P{X < a}for all real numbers a.