Chapter 1
Probability Concepts
1.1 The given sets are:
A = {1,2,3,4} B = {0,1,2,3,4,5,6,7,8}
C = {x | x real and 1≤ x <3} D = {2,4,7} E = {4,7,8,9,10}.
We observe that:
1.2 By inspection,
A is finite and countable.
B is finite and countable.
C is infinite and uncountable.
D is finite and countable.
E is finite and countable.
(a)
BA I = A = {1,2,3,4}.
(b)
EDBA
UUU
= {1,2,3,4,5,6,7,8,9,10}.
(c)
(
DEB
U )
I
= D = {2,4,7}.
(d)
EB −
= {1,2,3,5,6}.
(e)
EDBA
III
={4}.
1.3 The universal set is U = {0,1,2,3,4,5,6,7,8,9,10,11,12}. The subsets are
A = {0,1,4,6,7,9}, B = {2,4,6,8,10,12} and C = {1,3,5,7,9,11}. By inspection,
(a)
BA I = {4,6}.
(b) (
BA U ) CI =
CA I = {1,7,9}.
1
2
Signal Detection and Estimation
(c)
CB U = {0}.
(d)
AB −
= {0,1,7,9}.
(e)
(
BA
U
)
(
CA
U
)
I
=
A
(
CB
I
)
U
= A = {0,1,4,6,7,9}.
(f )
CA I = {0,4,6}.
(g) −B
=C ∅.
(h)
CB I = B = {2,4,6,8,10,12}.
1.4 Applying the definitions, we have
(a)
BA −
U A
B
III )(
DCBAc
U
A B
C D
B
(b)
(
CBA
U )
I
U
A
C
Ad)(
U
A
(e) A∩B
U
A B
Probability Concepts
1.5 This is easily verified by Venn diagrams. Hence, we have
BA ⊂ and
CB ⊂ , then
B
A
C
B
U
B
C
CA ⊂
A
3
1.6 By inspection, B and C are mutually exclusive.
1.7 Let R, W and B denote red ball drawn, white ball drawn and blue ball drawn,
respectively
10R , 3W,
7B
(a)
=RP
(
)
(b)
=WP
(
)
balls
red of
Number
Total
of
balls
number
3
20
15.0
=
.
10
73
++
=
10
=
1
2
=
5.0
.
=
35.0
.
(c)
(e)
)
=BP
(
7
20
=WRP U
(
)
3
10
+
20
=
13
20
=
65.0
.
(d)
RP
(
1)
−=
RP
(
)
=
1
2
=
5.0
.
B1 ≡ first ball drawn is blue.
W2 ≡ 2nd ball drawn is white.
R3 ≡ 3rd ball drawn is red.
1.8 Let
and hence,
(a) The ball is replaced before the next draw ⇒ the events are independent
1
RWBP
(
I
I
2
)
3
|
3
1
1
2
2
|
)
(
)
)
=
1
RPWPBP=
(
210
8000
WBRPBWPBP
(
(
7
20
()
(
)
1
3
10
20
20
02625
.0
=
=
=
I
3
)
2
4
Signal Detection and Estimation
(b) Since the ball is not replaced, the sample size changes and thus, the
events are dependent. Hence,
RWBP
(
1
I
2
I
1.9
)
=
3
=
|
2
1
)
(
WBRPBWPBP
(
7
20
(
10
18
)
1
3
19
0307
.0
=
I
1
3
|
)
2
10R , 3W
B1
7B
2R , 6W
B2
1B
Let R1 and R2 denote draw a red ball from box B1 and B2 respectively, and let
W1 and W2 denote also draw a white ball from B1 and B2.
=
RRPRP
(
1
(
)
2
1
|
)
=
RPRP
(
(
)
1
2
)
since
the events are
(a)
)
2
independent. Hence,
RP
(
1
R
I
RP
(
1
=R
2
)
I
10
20
2
9
=
1
9
=
.0
111
.
(b) Similarly,
WWP
(
2
1
I
)
=
WPWP
(
2
(
)
1
)
=
3
20
6
9
=
1.0
(c) Since we can have a different color from each box separately, then
BWP
(
I
)
=
BWP
(
1
I
)
+
2
BWP
(
1
2
I
)
=
3
20
1
9
+
6
9
7
20
=
25.0
.
1.10 Let B1 and B2 denote Box 1 and 2 respectively. Let B denote drawing a black
ball and W a white ball. Then ,
B1
4W , 2B
B2
3W , 5B
Probability Concepts
5
Let B2 be the larger box, then P(B2) = 2P(B1). Since
BP
(
)
+
BP
(
1
2
1)
=
, we
obtain
BP
(
1
)
=
1
3
and
BP
(
2
)
=
2
3
.
(a) P(1B | B2) =
=
.0
625
.
5
8
2
6
(b) P(1B | B1) =
(c) This is the total probability of drawing a black ball. Hence
3333
.0
=
.
BP
)1(
=
=
2
BPBBP
|1(
2
5
8
3
(
1
3
)
2
6
+
=
)
2
.0
5278
.
+
BPBBP
|1(
1
(
)
1
)
(d) Similarly, the probability of drawing a white ball is
BPBWPWP
)1(
=
(
|
)
+
BPBWP
1(
1
(
)
1
|
=
1(
3
8
)
1
3
2
4
6
2
3
+
2
.0
=
4722
.
)
1.11 In four tosses:__ __ __ __, we have three 1s and one is not 1. For example
111 . Hence, the probability is
1
1
6
1
6
1
6
5
6
=
1
6
3
5
6
but we have
4
3
ways of
obtaining this. Therefore, the probability of obtaining 3 ones in 4 tosses is
!4
!1!3
1
6
3
5
6
=
.0
01543
.
1.12 Let R, W and G represent drawing a red ball, a white ball, and a green ball
respectively. Note that the probability of selecting Urn A is P(Urn A) = 0.6, Urn B
is P(Urn B) = 0.2 and Urn C is P(Urn C) = 0.2 since
P(Urn A)+P(Urn B)+P(Urn C) =1.
(a)
(P 1W | Urn B) =
WP
1(
P
Urn
I
B
)
(Urn
B
)
=
30
100
=
3.0
.
(b)
(P 1G | Urn B) =
(c) P(Urn C | R) =
P
.
40
4.0
=
100
I
RC
(Urn
RP
)
(
)
. Also,
6
Signal Detection and Estimation
P(R | Urn C) =
P
(Urn
P
I
RC
C
(Urn
)
)
⇒
P
(Urn
RC
|
)
=
RP
(
|
Urn
PC
)
RP
)
(
(Urn
C
)
.
We need to determine the total probability of drawing a red ball, which is
RP
(
)
=
=
Thus,
P
(Urn
Urn
)
|
(
6.0
RP
(
30
100
=RC
|
)
(Urn
(
2.0
A
)
)
+
PA
)
30
100
+
)2.0()4.0(
|
RP
(
+
40
(
2.0
100
25.0
.
=
32.0
PB
)
(Urn
B
)
+
RP
(
|
Urn
PC
)
(Urn
C
)
Urn
)
=
32.0
1.13 In drawing k balls, the probability that the sample drawn does not contain a
particular ball in the event Ei, i = 0, 1,2, … , 9, is
EP
(
)
i
=
EEP
(
i
j
)
M
9
10
=
k
8
10
k
(a) P(A) = P(neither ball 0 nor ball1) = P(E0E1) =
8
10
(b) P(B) = P( ball 1 does not appear but ball 2 does)
k
k
.
=
EP
(
1
)
−
EEP
(
1
2
)
=
k
9
10
k
−
k
8
10
k
=
9
k
k
8
−
k
10
.
−
k
7
10
k
=
8
k
k
7
−
k
10
.
(c) P(AB) =
0 EEEP
(
1
2
)
=
EEP
(
1
0
)
−
EEEP
(
1
0
(d)
BAP
(
U
)
=
AP
)
(
+
BP
(
)
−
ABP
(
)
=
k
9
1.14 We have
k
8
−
10
k
)
=
2
k
+
7
f
X
x
)(
=
−
x
e
1
2
0
−δ+
x
(
1
2
,)3
x
≥
0
,
x
<
0
k
8
10
.
k
Probability Concepts
7
(a)
fx(x)
1/2
(1/2)
. . x
0 1 2 3
∞
=
∫
0
)(x
x
−
e
+
1
2
xδ
(
1[
2
is a density function.
)]3
dx
−
=
∞
∫
0
1
2
−
x
e
dx
+
∞
∫
0
1
2
x
(δ
−
)3
dx
=
1
2
+
1
2
=
1
.
f
X
∞
∫
0
Hence,
dxx
)(
f X
(a) P(X = 1) = 0 (the probability at a point of a continuous function is zero).
=XP
(
)3
=
XP
(
≥
)1
=
1
2
∞
∫
1
=
5.0
.
f
X
dxx
)(
1.15
+=
1
2
∞
∫
1
1
2
−
x
e
dx
=
1
−
+
e
)
(
1
1
2
=
.0
6839
.
fX(x)
1/4
. . . . x
1/8
-3 -2 -1 0 1 2 3
(a) The cumulative distribution function of X for all the range of x is,
du
=
1
8
x
+
3
8
for
−≤≤−
3
x
1
,
x
1
∫
8
3
−
1
2
xF
)(
X
=
x
∫
∞−
f
X
duu
)(
=
x
and ∫
1
−
3
4
and
1
4
du
+
1
4
=
x
+ ∫
1
1
8
du
=
1
4
x
8
x
+
for
1
≤≤−
x
1
,
+
5
8
for
1
≤≤
x
3
,
8
and
xFX
1)(
=
for
Signal Detection and Estimation
x
≥
3
.
Thus,
FX (x) =
x
+
0
3
1
8
8
1
1
4
2
1
5
8
8
1
+
+
x
x
,
x
−<
3
,
1
−<≤−
3
x
,
1
<≤−
x
1
,
1
<≤
x
3
,
x
≥
3
(b) Calculating the area from the graph, we obtain
1.16 The density function is as shown
12)1
4
=