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Barnett/Ziegler/Byleen
College Algebra: A Graphing Approach
Chapter Five
Systems: Matrices
Copyright © 2000 by the McGraw-Hill Companies, Inc.
(A) 2x – 3y = 2
Nature of Solutions to Systems
of Equations
x + 2y = 8
y
5
(4, 2)
y
(B) 4x + 6y = 12
2x + 3y = –6
x
–5
5
5
–5
x
–5
Lines intersect at one point only.
Exactly one solution: x = 4, y = 2
5
–5
y
Lines are parallel (each has slope –2/3).
No solution
5
.
(C) 2x – 3y = –6
–x +
3/
2
y =3
x
–5
5
–5
Lines coincide. Infinitely many solutions.
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-1-44
Systems of Linear Equations: Basic Terms
A system of linear equations is consistent if it has one or more solutions and
inconsistent if no solutions exist. Furthermore, a consistent system is said to
be independent if it has exactly one solution (often referred to as the unique
solution) and dependent if it has more than one solution.
Possible Solutions to a Linear System
The linear system
ax + by = h
cx + dy = k
must have:
1. Exactly one solution
or
2. No solution
or
3. Infinitely many solutions
[Consistent and independent]
[Inconsistent]
[Consistent and dependent]
There are no other possibilities.
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-1-45
Augmented Matrix
In general, associated with each linear system of the form
a11 x1 + a12 x2 = k1
a2 1 x1 + a22 x2 = k2
where x1 and x2 are variables, is the augmented matrix of the system:
Column
Column11(C
(C
1) )
1
Column22(C
(C
Column
2)2)
Column33(C
(C
)
Column
3)3
a11

a21
a 12
a 22
k1

k 2 
Row
Row1 1( (RR11) )
Row2 2( (RR2) )
Row
2
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-2-46
Elementary Row Operations Producing
Row-Equivalent Matrices
An augmented matrix is transformed into a row-equivalent matrix if any
of the following row operations is performed:
1. Two rows are interchanged (Ri  Rj).
2. A row is multiplied by a nonzero constant (kRi  Ri).
3. A constant multiple of another row is added to a given row (kRj + Ri  Ri).
[Note: The arrow means "replaces."]
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-2-47
Reduced Matrix
A matrix is in reduced form if:
1.
Each row consisting entirely of 0’s is below any row
having at least one nonzero element.
2.
The leftmost nonzero element in each row is 1.
3.
The column containing the leftmost 1 of a given row has
0’s above and below the 1.
4.
The leftmost 1 in any row is to the right of the leftmost 1
in the preceding row.
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-3-48
Gauss-Jordan Elimination
Step 1.
Choose the leftmost nonzero column and use appropriate row
operations to get a 1 at the top.
Step 2.
Use multiples of the row containing the 1 from Step 1 to get
zeros in all remaining places in the column containing this 1.
Step 3.
Repeat Step 1 with the submatrix formed by (mentally) deleting
the row used in Step 2 and all rows above this row.
Step 4.
Repeat Step 2 with the entire matrix, including the mentally
deleted rows. Continue this process until it is impossible to
go further.
[Note: If at any point in the above process we obtain a row with all 0’s
to the left of the vertical line and a nonzero number n to the right, we
can stop, since we will have a contradiction: 0 = n, n  0. We can then
conclude that the system has no solution.]
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-3-49
Matrix Product
3  2
2  3
 2

 –2
3
1



–1

2
1
2
–1
3
0
2
=
1
[2 3 –1]
[–2 1 2]
2
3
[2 3 –1]
0
–1
2
1
3
2
[–2 1 2] 0
–1
2
Copyright © 2000 by the McGraw-Hill Companies, Inc.
2  2
 9

 –2
=
4

–2
5-5-50
Inverse of a Square Matrix
If M is a square matrix of order n and if there exists a matrix M –1
(read "M inverse") such that
M –1M = MM –1 = I
then M –1 is called the multiplicative inverse of M or, more simply,
the inverse of M.
Finding the Inverse if it Exists
If [M | I ] is transformed by row operations into [I | B], then the
resulting matrix B is M –1. If, however, we obtain all 0's in one or
more rows to the left of the vertical line, then M –1 does not exist.
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-5-51
Basic Properties of Matrices
Assume all products and sums are defined for the indicated matrices A, B, C, I,
and 0.
Addition Properties
ASSOCIATIVE:
(A + B) + C = A + (B + C)
COMMUTATIVE:
A + B = B + A
ADDITIVE IDENTITY:
A + 0 = 0 + A = A
ADDITIVE INVERSE:
A + (–A) = (–A) + A = 0
Multiplication Properties
ASSOCIATIVE:
A(BC) = (AB)C
MULTIPLICATIVE IDENTITY:
AI = IA = B
MULTIPLICATIVE INVERSE:
If A is a square matrix and A–1 exists, then
AA–1 = A–1A = I .
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-6-52(a)
Basic Properties of Matrices
Assume all products and sums are defined for the indicated matrices A, B, C, I,
and 0.
Combined Properties
LEFT DISTRIBUTIVE:
A(B + C) = AB + AC
RIGHT DISTRIBUTIVE:
(B + C)A = BA + CA
Equality
ADDITION:
If A = B, then A + C = B + C.
LEFT MULTIPLICATION:
If A = B, then CA = CB.
RIGHT MULTIPLICATION:
If A = B, then AC = BC.
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-6-52(b)
Using Inverse Methods to Solve
Systems of Equations
If the number of equations in a system equals the number of
variables and the coefficient matrix has an inverse, then the
system will always have a unique solution that can be found by
using the inverse of the coefficient matrix to solve the
corresponding matrix equation.
Matrix equation
Solution
AX = B
X = A–1B
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-6-53
Graph of a Linear Inequality
y
y =2x -3
y
8
–5
(4, y ) y > 2(4) - 3 = 5;
point in upper half-plane
(4, y ) y = 2(4) - 3 = 5;
point on line
(4, y ) y < 2(4) - 3 = 5;
point in lower half-plane
5
10
0
x
(a) y  2x – 3
x
0
(b) y > 2x – 3
x
y
0
–8
y
(c) y  2x – 3
Copyright © 2000 by the McGraw-Hill Companies, Inc.
y
x
0
x
(d) y < 2x – 3
5-7-54
Procedure for Graphing Linear Inequalities
Step 1. Graph Ax + By = C as a dashed line if equality is not
included in the original statement or as a solid line if
equality is included.
Step 2. Choose a test point anywhere in the plane not on the
line and substitute the coordinates into the inequality.
The origin (0,0) often requires the least computation.
Step 3. The graph of the original inequality includes the halfplane containing the test point if the inequality is
satisfied by that point, or the half-plane not containing
that point if the inequality is not satisfied by the point.
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-7-55
Solution of Linear Programming Problems
Step 1. Form a mathematical model for the problem:
(A) Introduce decision variables and write a linear
objective function.
(B) Write problem constraints in the form of linear
inequalities.
(C) Write nonnegative constraints.
Step 2. Graph the feasible region and find the corner points.
Step 3. Evaluate the objective function at each corner point
to determine the optimal solution.
Copyright © 2000 by the McGraw-Hill Companies, Inc.
5-8-56