Name
Math 311
Exam 1 Version B
Section 502
Spring 2015
Solutions
P. Yasskin
Points indicated. Show all work.
30 points
1.
1
/30
5
/20
2
/10
6
/15
3
/10
7
/15
4
/10 Total
/110
Solve the traffic flow system shown
at the right. Find the smallest non-negative values
of w, x, y, z. In your augmented matrix, keep
the variables in the order w, x, y, z.
Solution: The equations are:
w
100
x
300
w
x
x
400
y
200
x
y
y
100
z
200
y
z
z
300
w
200
w
200
200
100
z
100
The augmented matrix and row operations are:
1
1
0
0
200
1
1
0
0
200
0
1
1
0
200
0
1
1
0
200
0
0
1
1
100
0
0
1
1
100
1
0
0
1
100
0
1
0
1
100
R4
R1
R1
R2
R4
R2
1 0
1
0
0
R1
R3
1 0 0
1
100
w
r
100
0 1
1
0
200
R2
R3
0 1 0
1
100
x
r
100
0 0
1
1
100
0 0 1
1
100
y
r
100
0 0
1
1
100
0 0 0
0
0
z
r
R4
R3
For the smallest non-negative solution, we take r 100:
w 200
x 0
y 200
z 100
1
2.
By definition, a matrix, A, is idempotent if A 2
10 points
Prove if A is idempotent, then 1
Solution:
Thus 1
3.
1
A
A is non-singular and 1
A.
1
A
1
1 A.
2
1A
1 1 A A 1 A 2 1 1 A A 1 A 1 since A 2
2
2
2
2
2
1 A are inverses and 1 A is invertible and non-singular.
2
1
A and 1
10 points
If A is a 50
AB C
AB AC.
60 matrix while B and C are 60
A.
80 matrices, prove
HINT: Prove equality of the ij-component of each side.
60
Solution:
AB
C
60
A ik B
ij
C
kj
k 1
AB
ij
AC
ij
AB
These are equal. So A B
4.
10 points
AC
C
60
A ik B kj
k 1
A ik C kj
k 1
ij
AB
AC
A matrix A satisfies E 1 E 2 E 3 A
0 1 0
C kj
k 1
0 0 1
E1
60
A ik B kj
B where
1 0 0
E2
1 0 0
E3
0 1 0
1 0 0
0 0 2
5 3
B
0 1 0
4 0 1
0
2
0
0
2
and the ’s represent unknown non-zero numbers. Find det A.
Solution: det E 1
det A
1
det B
det E 1 det E 2 det E 3
det E 2
2
20
1 2 1
det E 3
1
det B
5 2
2
20
10
2
5.
20 points
Phosphoric acid H 3 PO 4 combines with sodium hydroxide NaOH to produce trisodium
phosphate Na 3 PO 4 and water H 2 O according to the chemical equation:
a H 3 PO 4 b NaOH
c Na 3 PO 4 d H 2 O
To balance this chemical equation, you must solve the system
H:
3a
P:
O:
4a
Na :
b
2d
a
c
b
4c
b
3c
d
a. Write out the augmented matrix for this system of 4 equations in 4 unknowns. DO NOT SOLVE.
Solution:
3 1
0
2
0
1 0
1
0
0
4 1
4
1
0
0 1
3
0
0
b. Compute the determinant of the matrix of coefficients.
Solution: Use 2 row operations. Then expand on column 2:
det A
3 1
0
2
3
1
0
2
1 0
1
0
1
0
1
0
4 1
4
1
R3
R1
1
0
4
1
0 1
3
0
R4
R1
3 0
3
2
1
1
1 0
1
4 1
3
3 2
Add column 1 to column 2. Then expand on row 1:
det A
1
1
0
0
1
3 1
3
6 2
1 1
3 1
6 2
1 1
6
6
0
c. What property of the augmented matrix says there is at least one solution? (Circle one.)
i.
ii.
iii.
iv.
The fact that the determinant of the matrix of coefficients is non-zero.
The fact that the determinant of the matrix of coefficients is zero.
The fact that the system is homogeneous (the right hand sides are all zero).
The fact that the matrix of coefficients is square 4 4 .
CORRECT
d. What additional property says there are infinitely many solutions? (Circle one.)
i.
ii.
iii.
iv.
The fact that the determinant of the matrix of coefficients is non-zero.
The fact that the determinant of the matrix of coefficients is zero.
CORRECT
The fact that the system is homogeneous (the right hand sides are all zero).
The fact that the matrix of coefficients is square 4 4 .
3
1 2 a
6.
15 points
Let A
. Given that det A
4 3 b
4, determine each of the following:
c d 0
1
c 2
4
3
b
c
d
0
det 3A
7.
d a
4
108
det A
1
1
6
a
4
9
b
2 1 a
12
4
3 4 b
c 3d 0
1
4
d c 0
15 points
For each of the following sets with operations, determine whether it forms a vector
space. If it does, just say "Yes". If it does not, say "No" and give an axiom or other property it violates
and show why.
a. The set of infinite sequences, S
a
b
a1
b1, a2
b2,
, an
a
bn,
a1, a2, , an,
, with
and
a
a1, a2, , an,
Solution: Yes, it is a vector space.
b. The set of traceless 2
A
B
2 matrices M 0 2, 2
A 11
B 11 A 12
B 12
A 21
B 21 A 22
B 22
and
Solution: No, it violates A 8 since 1
c. The set of traceless 2
A
B
A
B 22 A 12
B 12
A 21
B 21 A 22
B 11
and
Solution: No, it violates A 1 since B
A
A 21 A 22
A 22
A 12
A 21
A 11
A 22 A 12
A
A
A
A 11
A 22
0
A 11
A 22
0
with
A
A 21 A 11
2 matrices M 0 2, 2
A 11
A 11 A 12
A
A 11 A 12
A 21 A 22
A 11
A 12
A 21
A 22
B 11
A 22 B 12
A 12
B 21
A 21 B 22
A 11
A
B
4