Showing all intermediate results, calculate the following
expressions or give reasons why they are undefined:
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21. General rules. Prove (2) for matrices
and a general scalar.
22. Product. Write AB in Prob. 11 in terms of row and
column vectors.
23. Product. Calculate AB in Prob. 11 columnwise. See
Example 1.
24. Commutativity. Find all matrices
that commute with , where
25. TEAM PROJECT. Symmetric and Skew-Symmetric
Matrices. These matrices occur quite frequently in
applications, so it is worthwhile to study some of their
most important properties.
(a) Verify the claims in (11) that for a
symmetric matrix, and for a skew-
symmetric matrix. Give examples.
(b) Show that for every square matrix C the matrix
is symmetric and is skew-symmetric.
Write C in the form , where S is symmetric
and T is skew-symmetric and find S and T in terms
of C. Represent A and B in Probs. 11–20 in this form.
(c) A linear combination of matrices A, B, C,, M
of the same size is an expression of the form
(14)
where a,, m are any scalars. Show that if these
matrices are square and symmetric, so is (14); similarly,
if they are skew-symmetric, so is (14).
(d) Show that AB with symmetric A and B is symmetric
if and only if A and B commute, that is,
(e) Under what condition is the product of skew-
symmetric matrices skew-symmetric?
26–30
FURTHER APPLICATIONS
26. Production. In a production process, let N mean “no
trouble” and T “trouble.” Let the transition probabilities
from one day to the next be 0.8 for , hence 0.2
for , and 0.5 for , hence 0.5 for T : T.T : NN : T
N : N
AB BA.
Á
aA bB cC
Á
mM,
Á
C S T
C C
T
C C
T
a
kj
a
jk
a
kj
a
jk
b
jk
j k.B 3b
jk
4
A 3a
jk
42 2
B 3b
jk
4, C 3c
jk
4,
A 3a
jk
4,2 2
b
T
Ab,
aBa
T
,
aCC
T
,
C
T
ba
Ab Bb(A B)b,1.5a 3.0b,
1.5a
T
3.0b,
ab,
ba,
aA,
Bb
ABC,
ABa,
ABb,
Ca
T
BC,
BC
T
,
Bb,
b
T
B
b
T
A
T
Aa,
Aa
T
,
(Ab)
T
,
(3A 2B)
T
a
T
3A
T
2B
T
,3A 2B,
(3A 2B)
T
,
CC
T
,
BC,
CB,
C
T
B
AA
T
,
A
2
,
BB
T
,
B
2
AB,
AB
T
,
BA,
B
T
A
SEC. 7.2 Matrix Multiplication 271
If today there is no trouble, what is the probability of
N two days after today? Three days after today?
27. CAS Experiment. Markov Process. Write a program
for a Markov process. Use it to calculate further steps
in Example 13 of the text. Experiment with other
stochastic matrices, also using different starting
values.
28. Concert subscription. In a community of 100,000
adults, subscribers to a concert series tend to renew their
subscription with probability and persons presently
not subscribing will subscribe for the next season with
probability . If the present number of subscribers
is 1200, can one predict an increase, decrease, or no
change over each of the next three seasons?
29. Profit vector. Two factory outlets and in New
York and Los Angeles sell sofas (S), chairs (C), and
tables (T) with a profit of , and , respectively.
Let the sales in a certain week be given by the matrix
SCT
Introduce a “profit vector” p such that the components
of give the total profits of and .
30. TEAM PROJECT. Special Linear Transformations.
Rotations have various applications. We show in this
project how they can be handled by matrices.
(a) Rotation in the plane. Show that the linear
transformation with
is a counterclockwise rotation of the Cartesian -
coordinate system in the plane about the origin, where
is the angle of rotation.
(b) Rotation through n
. Show that in (a)
Is this plausible? Explain this in words.
(c) Addition formulas for cosine and sine. By
geometry we should have
Derive from this the addition formulas (6) in App. A3.1.
c
cos (a b)
sin (a b)
sin (a b)
cos (a b)
d
.
c
cos a
sin a
sin a
cos a
dc
cos b
sin b
sin b
cos b
d
A
n
c
cos nu
sin nu
sin nu
cos nu
d
.
u
x
1
x
2
A
c
cos u
sin u
sin u
cos u
d
,
x
c
x
1
x
2
d
,
y
c
y
1
y
2
d
y Ax
F
2
F
1
v Ap
A
c
400
100
60
120
240
500
d
F
1
F
2
$30$35, $62
F
2
F
1
0.2%
90%
3 3