Download Elementary Linear Algebra

Elementary Linear Algebra
Howard Anton ＆ Chris Rorres
1
Chapter Contents
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1.1 Introduction to System of Linear
Equations
1.2 Gaussian Elimination
1.3 Matrices and Matrix Operations
1.4 Inverses; Rules of Matrix Arithmetic
1.5 Elementary Matrices and a Method for
Finding A1
1.6 Further Results on Systems of Equations
and Invertibility
1.7 Diagonal, Triangular, and Symmetric
Matrices
2
1.1 Introduction to
Systems of Equations
3
Linear Equations

Any straight line in xy-plane can be
represented algebraically by an equation of
the form:
a1 x  a2 y  b

General form: define a linear equation in the n
variables x1 , x2 ,..., xn :
a1x1  a2 x2  ...  an xn  b


Where a1 , a2 ,..., an , and b are real constants.
The variables in a linear equation are sometimes
called unknowns.
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Example 1
Linear Equations
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1
The equations x  3 y  7, y  x  3 z  1, and
x1  2 x2  3x3  x4  7 are linear. 2
Observe that a linear equation does not involve any
products or roots of variables. All variables occur only to
the first power and do not appear as arguments for
trigonometric, logarithmic, or exponential functions.
The equations x  3 y  5, 3x  2 y  z  xz  4, and y  sin x
are not linear.

A solution of a linear equation is a sequence of n numbers
s1 , s2 ,..., sn such that the equation is satisfied. The set of
all solutions of the equation is called its solution set or
general solution of the equation
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Example 2
Finding a Solution Set (1/2)


Find the solution of
(a ) 4 x  2 y  1
Solution(a)
we can assign an arbitrary value to x and solve for y ,
or choose an arbitrary value for y and solve for x .If
we follow the first approach and assign x an arbitrary
1
1
value ,we obtain x  t , y  2t  1
or
x

t

, y  t2
1
1
2
2


2
arbitrary numbers t1, t 2 are called parameter.
for example
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t1  3 yields the solution x  3, y 
2
4
as t 2 
11
2
6
Example 2
Finding a Solution Set (2/2)


Find the solution of (b) x1  4 x2  7 x3  5.
Solution(b)
we can assign arbitrary values to any two
variables and solve for the third variable.
 for example
x1  5  4s  7t ,

x2  s,
x3  t
where s, t are arbitrary values
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Linear Systems (1/2)
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A finite set of linear equations in
the variables x1 , x2 ,..., xn
a11x1  a12 x2  ...  a1n xn  b1
is called a system of linear
a21x1  a22 x2  ...  a2 n xn  b2
equations or a linear system .
A sequence of numbers
s1 , s2 ,..., sn is called a solution
of the system.
A system has no solution is said
to be inconsistent ; if there is at
least one solution of the system,
it is called consistent.
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


am1 x1  am 2 x2  ...  amn xn  bm
An arbitrary system of m
linear equations in n unknowns
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Linear Systems (2/2)


Every system of linear equations has either
no solutions, exactly one solution, or
infinitely many solutions.
A general system of two linear equations:
(Figure1.1.1) a1 x  b1 y  c1 (a1 , b1 not both zero)
a2 x  b2 y  c2 (a2 , b2 not both zero)
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

Two lines may be parallel -> no solution
Two lines may intersect at only one point
-> one solution
Two lines may coincide
-> infinitely many solution
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Augmented Matrices


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The location of the +’s,
the x’s, and the =‘s can
be abbreviated by writing
only the rectangular array
of numbers.
This is called the
augmented matrix for the
system.
Note: must be written in
the same order in each
equation as the unknowns
and the constants must be
on the right.
a11x1  a12 x2  ...  a1n xn  b1
a21 x1  a22 x2  ...  a2 n xn  b2




am1 x1  am 2 x2  ...  amn xn  bm
1th column
a11 a12 ... a1n b1 
a a ... a

b
2n
2 
 21 22
  

 


a
a
...
a
b
mn
m
 m1 m 2
1th row
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Elementary Row Operations
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
The basic method for solving a system of linear equations is to
replace the given system by a new system that has the
same solution set but which is easier to solve.
Since the rows of an augmented matrix correspond to the
equations in the associated system. new systems is generally
obtained in a series of steps by applying the following three
types of operations to eliminate unknowns systematically. These
are called elementary row operations.
1. Multiply an equation through by an nonzero constant.
2. Interchange two equation.
3. Add a multiple of one equation to another.
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Example 3
Using Elementary row Operations(1/4)
x  y  2z  9
2 x  4 y  3z  1
3x  6 y  5 z  0
1 1 2 9 
2 4  3 1


3 6  5 0
add - 2 times
the first equation
to the second
x  y  2z  9
2 y  7 z  1 7
 3x  6 y  5 z 
add - 2 times
the first row
to the second


0
add -3 times
the first equation
to the third

9 
1 1 2
0 2  7  17


3 6  5 0 
add -3 times
the first row
to the third

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Example 3
Using Elementary row Operations(2/4)
add -3 times
x  y  2 z  9 multiply the second x  y  2 z  9
1
the second equation
equation
by
7
17
2 y  7 z  17
y 2 z  2
to the third
2








3 y  11z  27
3 y  11z  0
multily the second
9 
1 1 2
1
0 2  7  17 
row by

  2 

0 3  11  27
9  add -3 times
1 1 2
0 1  7  17  the second row
2
2 

to the third
0 3  11  27 
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Example 3
Using Elementary row Operations(3/4)
x  y  2z 
9
Multiply the third
equation by -2
x  y  2z 
9
7
17
y

z


y  72 z   172 
2
2

z 3
 12 z   32
1 1 2
0 1  7
2

0 0  12
9 
 172 
 32 
Multily the third
row by -2

1 1 2
0 1  7
2

0 0 1
Add -1 times the
second equation
to the first

9  Add -1 times the
 172  second row
to the first

3  
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Example 3
Using Elementary row Operations(4/4)
x
 112 z 
35
2
y  72 z   172
z
1 0 112
0 1  7
2

0 0 1
3



 
3 
35
2
17
2
Add - 121 times
the third equation
to the first and 72 times
the third equation
to the second
Add - 121 times
the third row
to the first and 72
times the third row
to the second
   

x
y
1
 2
z 3
1 0 0 1
0 1 0 2


0 0 1 3
 The solution x=1,y=2,z=3 is now evident.
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Exercise Set 1.1
Question 3
Add - 121 times
the third equation
to the first and 72 times
the third equation
to the second


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Exercise Set 1.1
Question 8
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Exercise Set 1.1
Question 11
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