Solutions Manual
to accompany
Probability,
Random Variables
Stochastic Processes
and
Fourth Edition
Athanasios Papoulis
Polytechnic University
S. Unnikrishna Pillai
Polytechnic University
Solutions Manual to accompany
PROBABILITY, RANDOM VARIABLES AND STOCHASTIC PROCESSES, FOURTH EDITION
ATHANASIOS PAPOULIS
Published by McGraw-Hill Higher Education, an imprint of The McGraw-Hill Companies, Inc., 1221 Avenue of the Americas,
New York, NY 10020. Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
The contents, or parts thereof, may be reproduced in print form solely for classroom use with PROBABILITY, RANDOM
VARIABLES AND STOCHASTIC PROCESSES, FOURTH EDITION, provided such reproductions bear copyright notice, but may
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CHAPTER 2
2 - 1
- -
We use D e M o r g a n ' s l a w :
(a) X + 6 + I + B = AB + A% = A ( B + % ) = A
because
= (01 BB = I01
2-2
2-3
-
If A = { 2 < x ; 5 )
- -
A + B = { 2 < x < 6 )
- -
(A+B)(E)
= { 2 < x < 6 1
-
= { 2 < x < 3 )
B = { 3 < x < 6 1
AB = { 3 < x < 5 )
- -
- -
[ { x < 3 1 + E x > 5 1 ]
+ { 5 < x < 6 1
-
S = {-=-
2-6
Any s u b s e t of S c o n t a i n s a countable number o f elements, hence, i t
can be w r i t t e n a s a countable union of elementary e v e n t s . It i s
t h e r e f o r e a n event.
2-7
Forming a l l unions, i n t e r s e c t i o n s , and complements of t h e s e t s E l )
and {2,3), we o b t a i n t h e following sets:
(01, C11, (41, {2,31, {1,41, {1,2,31, {2,3,41, {1,2,3,41
2-8
I f ACB,P(A) = 114, and P(B) = 113, t h e n
2-10 We use i n d u c t i o n . The formula i s t r u e f o r n = 2 because
P(A1A2) - P ( A ~ I A ~ ) P ( A ~ ) .
Suppose t h a t i t i s t r u e f o r n. Since
we conclude t h a t i t must be t r u e f o r n + l .
2-11 F i r s t s o l u t i o n . The t o t a l number of m element s u b s e t s e q u a l s (")
m
( s e e
The t o t a l number of m element s u b s e t s c o n t a i n i n g 5 e q u a l s
0
Probl. 2-26).
n- l
(m-l)
Hence
Second s o l u t i o n . C l e a r l y , P{C, I
is i n a s p e c i f i c Am. Hence ( t o t a l p r o b a b i l i t y )
A
~
)
= mln i s t h e p r o b a b i l i t y t h a t 5
0
where t h e summation i s over a l l sets A . m
2-12
2
(a) P E 6 < t < 8 1 = -
10
(b) ~ { 6
- -
- < t - < 81t > 51 = P ( t , 51
P E 6 r t s 8 1
2
= - 5
2-13
From (2-27) it follows t h a t
Equating t h e two s i d e s and s e t t i n g t l = t O + A t w e o b t a i n
f o r every to. Hence,
D i f f e r e n t i a t i n g t h e s e t t i n g c = a ( O ) , we conclude t h a t
2-14
I f A and B a r e independent, then P (AB) = P (A)P (B) . I f they a r e
mutually exclusive, then P(AB) = 0 , Hence, A and B a r e mutually
exclusive and independent i f f P(A)P(B) = 0.
C l e a r l y , A1 = A1A2 + ~
~
hence
i
i
~
I f t h e e v e n t s A
and
1
a r e independent, t h e n
2
hence, t h e e v e n t s A and A a r e independent. Furthermore, S i s
independent w i t h any A because SA = A. This y i e l d s
1
2
P(SA) = P(A) = P(S)P(A)
Hence, t h e theorem i s t r u e f o r n = 2 . To prove i t i n g e n e r a l we use
i n d u c t i o n : Suppose t h a t A
i s independent of A1, ..., A . C l e a r l y ,
a r e independent of B1, ... ,B . Therefore
n+l
n
-
An+l
and An+l
n
2.16 The desired probabilit,ies are given by (a)
(TI ;)
2.17 Let Al I A2 and Ad represent tire events
Al = "ball numbered less tha,n or equal to rn is drawn?
A2 = ('ball numbered rn i s drawn"
AS = ('ball numbered greater tillan rn is drawn"
P ( A 1 occu,rs nl = k - 1, A2 occurs n2 = 1 and A3 occurs n3 = 0 )
2.18 All cars are equally likely so that the first car is selected with
probability p = 113. This gives the desired probability to be
2.19 P{'drawing a white bad1 " } = &&
P("atleat one white ball i n k triu,ls ")
= 1 - P("all black balls in k trials")
2.20 Let D = 2r represent the penny diameter. So long as the center
of the penny is at a distance of r away from any side of the square,
the penny will be entirely inside the square. This gives the desired
probability to be
2.21 Refer to Exanlple 3.14.
( a ) Using (3.391, we get
(h)
P ( " t w o one-digit and four two-digit numbers1') =
2-22 The number of equations of the form P(AiAk) = P(Ai)P(Ak)
n
equals (*I.
The number of equations involving r sets equals (:).
number N of such equations equals
Hence the total
And since
we conclude that
2-23 We denote by B1 and B2 respectively the balls in boxes 1 and 2 and
by R the set of red balls. We have (assmption)
P(B1) = P(BZ) '0.5
Hence (Bayes' theorem)
P(R\B~) - 0.999
= 0.001
~ ( ~ 1 8 ~ )