Transcript Zeros of Polynomial Functions

10TH
EDITION
COLLEGE
ALGEBRA
LIAL
HORNSBY
SCHNEIDER
3.3 - 1
3.3
Zero of Polynomial Functions
Factor Theorem
Rational Zeros Theorem
Number of Zeros
Conjugate Zeros Theorem
Finding Zeros of a Polynomial Function
Descartes’ Rule of Signs
3.3 - 2
Factor Theorem
The polynomial x – k is a factor of the
polynomial (x) if and only if (k) = 0.
3.3 - 3
DECIDING WHETHER x – k IS A
FACTOR OF (x)
Example 1
Determine whether x – 1 is a factor of (x).
a. f ( x )  2 x 4  3 x 2  5 x  7
Solution By the factor theorem, x – 1 will be a
factor of (x) if and only if (1) = 0. Use synthetic
division and the remainder theorem to decide.
0 3 5 7
2 2
5 0
2 2 5
0 7
12
Use a zero
coefficient for
the missing
term.
(1) = 7
Since the remainder is 7 and not 0,
x – 1 is not a factor of (x).
3.3 - 4
Example 1
DECIDING WHETHER x – k IS A
FACTOR OF (x)
Determine whether x – 1 is a factor of (x).
b. f ( x )  3 x 5  2 x 4  x 3  8 x 2  5 x  1
Solution
2 1 8 5 1
3 1
2  6 1
3
1 2  6 1 0
13
(1) = 0
Because the remainder is 0, x – 1 is a factor.
Additionally, we can determine from the coefficients in
the bottom row that the other factor is
3.3 - 5
DECIDING WHETHER x – k IS A
FACTOR OF (x)
Example 1
Determine whether x – k is a factor of (x).
b. f ( x )  3 x 5  2 x 4  x 3  8 x 2  5 x  1
Solution
2 1 8 5 1
3 1
2  6 1
1 2  6 1 0
1 3
3
(1) = 0
3 x  x  2 x  6 x  1.
4
3
2
Thus, f ( x )  ( x  1)(3 x  x  2 x  6 x  1).
4
3
2
3.3 - 6
Example 2
FACTORING A POLYNOMIAL
GIVEN A ZERO
Factor the following into linear factors if –3 is
3
2
a zero of . f ( x )  6 x  19 x  2 x  3
Solution Since –3 is a zero of ,
x – (–3) = x + 3 is a factor.
3 6 19 2
18  3
6
1 1
3
3
0
Use synthetic division to
divide (x) by x + 3.
The quotient is 6x2 + x – 1.
3.3 - 7
Example 2
FACTORING A POLYNOMIAL
GIVEN A ZERO
Factor the following into linear factors if –3 is
3
2
a zero of . f ( x )  6 x  19 x  2 x  3
Solution x – (–3) = x + 3 is a factor.
The quotient is 6x2 + x – 1, so
f ( x )  ( x  3)(6 x  x  1)
2
f ( x )  ( x  3)(2 x  1)(3 x  1).
Factor 6x2 + x – 1.
These factors are all linear.
3.3 - 8
Rational Zeros Theorem
p
q
If
is a rational number written in
p
lowest terms, and if q is a zero of , a
polynomial function with integer
coefficients, then p is a factor of the
constant term and q is a factor of the
leading coefficient.
3.3 - 9
Proof
n
 p
 p
an    an 1  
q
q
n 1

 p
 a1    a0  0
q
p 
p 
an  n   an 1  n 1  
q 
q 
p
 a1    a0  0
q 
n 1
n
n 1
an p  an 1p q 
n
p  an p
n 1
 an 1p
n 2
q
 a1pq
 a1q
n 1
n 1
 a0q
n
  a q
0
n
Multiply by
qn; add
–a0 qn.
Factor
out p.
3.3 - 10
Proof
This result shows that –a0qn equals the
product of the two factors p and
(anpn–1 +  + a1qn–1).
For this reason, p must be a factor of –a0qn.
p
Since it was assumed that q is written in
lowest terms, p and q have no common
factor other than 1, so p is not a factor of qn.
Thus, p must be a factor of a0. In a similar
way, it can be shown that q is a factor of an.
3.3 - 11
Example 3
USING THE RATIONAL ZERO
THEOREM
Do the following for the polynomial function
4
3
2
defined by f ( x )  6 x  7 x  12 x  3 x  2.
a. List all possible rational zeros.
p
q
Solution For a rational number to be
zero, p must be a factor of a0 = 2 and q
must be a factor of a4 = 6. Thus, p can be
1 or 2, and q can be 1, 2, 3, or 6.
p
The possible rational zeros, q are,
1 1 1 2
1,  2,  ,  ,  ,  .
2 3 6 3
3.3 - 12
Example 3
USING THE RATIONAL ZERO
THEOREM
Do the following for the polynomial function
4
3
2
defined by f ( x )  6 x  7 x  12 x  3 x  2.
b. Find all rational zeros and factor (x) into
linear factors.
Solution Use the remainder theorem to show
that 1 is a zero.
1 6 7  12  3 2
Use “trial and
error” to find
6
13
1 2
zeros.
(1) = 0
6 13
1 2 0
The 0 remainder shows that 1 is a zero. The quotient is
6x3 +13x2 + x – 4, so (x) = (x – 1)(6x3 +13x2 + x – 2).
3.3 - 13
Example 3
USING THE RATIONAL ZERO
THEOREM
Do the following for the polynomial function
4
3
2
defined by f ( x )  6 x  7 x  12 x  3 x  2.
b. Find all rational zeros and factor (x) into
linear equations.
Solution Now, use the quotient polynomial
and synthetic division to find that –2 is a zero.
2 6 13 1  2
12  2
2
(–2 ) = 0
6 1 1
0
The new quotient polynomial is 6x2 + x – 1.
Therefore, (x) can now be factored.
3.3 - 14
Example 3
USING THE RATIONAL ZERO
THEOREM
Do the following for the polynomial function
4
3
2
defined by f ( x )  6 x  7 x  12 x  3 x  2.
b. Find all rational zeros and factor (x) into
linear equations.
Solution
f ( x )  ( x  1)( x  2)(6 x  x  1)
2
 ( x  1)( x  2)(3 x  1)(2 x  1).
3.3 - 15
Example 3
USING THE RATIONAL ZERO
THEOREM
Do the following for the polynomial function
4
3
2
defined by f ( x )  6 x  7 x  12 x  3 x  2.
b. Find all rational zeros and factor (x) into
linear equations.
Solution Setting 3x – 1 = 0 and 2x + 1 = 0
yields the zeros ⅓ and –½. In summary the
rational zeros are 1, –2, ⅓, –½, and the linear
factorization of (x) is
f ( x )  6 x 4  7 x 3  12 x 2  3 x  2
Check by
 ( x  1)( x  2)(3 x  1)(2 x  1).
multiplying
these factors.
3.3 - 16
Note In Example 3, once we obtained
the quadratic factor of 6x2 + x – 1, we were
able to complete the work by factoring it
directly. Had it not been easily factorable,
we could have used the quadratic formula
to find the other two zeros (and factors).
3.3 - 17
Caution The rational zeros theorem gives only
possible rational zeros; it does not tell us whether
these rational numbers are actual zeros. We must rely
on other methods to determine whether or not they are
indeed zeros. Furthermore, the function must have
integer coefficients. To apply the rational zeros theorem
to a polynomial with fractional coefficients, multiply
through by the least common denominator of all fractions.
For example, any rational zeros of p(x) defined below will
also be rational zeros of q(x).
1 3 2 2 1
1
p( x )  x  x  x  x 
6
3
6
3
4
q( x )  6 x 4  x 3  4 x 2  x  2
Multiply the terms of
p(x) by 6.
3.3 - 18
Fundamental Theorem of
Algebra
Every function defined by a polynomial
of degree 1 or more has at least one
complex zero.
3.3 - 19
Fundamental Theorem of Algebra
From the fundamental theorem, if (x) is of
degree 1 or more, then there is some
number k1 such that k1 = 0. By the factor
theorem,
f ( x )  ( x  k1)q1( x )
for some polynomial q1(x).
3.3 - 20
Fundamental Theorem of Algebra
If q1(x) is of degree 1 or more, the
fundamental theorem and the factor
theorem can be used to factor q1(x) in the
same way. There is some number k2 such
that q1(k2) = 0, so
q1( x )  ( x  k2 )q2 ( x )
3.3 - 21
Fundamental Theorem of Algebra
and
f ( x )  ( x  k1)( x  k2 )q2 ( x )
Assuming that (x) has a degree n and
repeating this process n times gives
f ( x )  a( x  k1)( x  k2 )
( x  kn ),
where a is the leading coefficient of (x).
Each of these factors leads to a zero of (x),
so (x) has the same n zeros k1, k2, k3,…, kn.
This result suggests the number of zeros
theorem.
3.3 - 22
Number of Zeros Theorem
A function defined by a polynomial of
degree n has at most n distinct zeros.
3.3 - 23
Example 4
FINDING A POLYNOMIAL FUNCTION
THAT SATISFIES GIVEN CONDITIONS
(REAL ZEROS)
Find a function  defined by a polynomial of
degree 3 that satisfies the given conditions.
a. Zeros of –1, 2, and 4; (1) = 3
Solution These three zeros give x – (–1) =
x + 1, x – 2, and x – 4 as factors of (x).
Since (x) is to be of degree 3, these are
the only possible factors by the number of
zeros theorem. Therefore, (x) has the form
f ( x )  a( x  1)( x  2)( x  4)
for some real number a.
3.3 - 24
Example 4
FINDING A POLYNOMIAL FUNCTION
THAT SATISFIES GIVEN CONDITIONS
(REAL ZEROS)
Find a function  defined by a polynomial of
degree 3 that satisfies the given conditions.
a. Zeros of –1, 2, and 4; (1) = 3
Solution To find a, use the fact that (1) = 3.
f (1)  a(1  1)(1  2)(1  4)
3  a(2)( 1)( 3)
3  6a
1
a
2
Let x = 1.
(1) = 3
Solve for a.
3.3 - 25
Example 4
FINDING A POLYNOMIAL FUNCTION
THAT SATISFIES GIVEN CONDITIONS
(REAL ZEROS)
Find a function  defined by a polynomial of
degree 3 that satisfies the given conditions.
a. Zeros of –1, 2, and 4; (1) = 3
Solution Thus,
or
1
f ( x )  ( x  1)( x  2)( x  4),
2
1 3 5 2
f ( x )  x  x  x  4. Multiply.
2
2
3.3 - 26
Example 4
FINDING A POLYNOMIAL FUNCTION
THAT SATISFIES GIVEN CONDITIONS
(REAL ZEROS)
Find a function  defined by a polynomial of
degree 3 that satisfies the given conditions.
b. –2 is a zero of multiplicity 3; (–1) = 4
Solution The polynomial function defined
by (x) has the form
f ( x )  a( x  2)( x  2)( x  2)
 a( x  2) .
3
3.3 - 27
Example 4
FINDING A POLYNOMIAL FUNCTION
THAT SATISFIES GIVEN CONDITIONS
(REAL ZEROS)
Find a function  defined by a polynomial of
degree 3 that satisfies the given conditions.
b. –2 is a zero of multiplicity 3; (–1) = 4
Solution Since (– 1) = 4,
f ( 1)  a( 1  2)
3
Remember:
(x + 2)3 ≠ x3 + 23
4  a(1)
3
a  4,
3
3
2
f
(
x
)

4(
x

2)

4
x

24
x
 48 x  32.
and
3.3 - 28
Note In Example 4a, we cannot clear
the denominators in (x) by multiplying both
sides by 2 because the result would equal
2 • (x), not (x).
3.3 - 29
Properties of Conjugates
For any complex numbers c and d,
c  d  c  d , cd  cd , and c   c  .
n
n
3.3 - 30
Conjugate Zeros Theorem
If (x) defines a polynomial function
having only real coefficients and if
z = a + bi is a zero of (x), where a
and b are real numbers,
then z  a  bi is also a zero of f ( x ).
3.3 - 31
Proof
Start with the polynomial function defined by
f ( x )  an x  an 1x
n
n 1

 a1x  a0 .
where all coefficients are real numbers. If
the complex number z is a zero of (x), then
n
n 1
f ( z )  an z  an 1z   a1z  a0  0.
Taking the conjugates of both sides of this
last equation gives
an z  an 1z
n
n 1

 a1z  a0  0.
3.3 - 32
Proof
Using generalizations of the properties
c  d  c  d and cd  cd gives
an z  an 1z
n 1

 a1z  a0  0
or an z n  an 1 z n 1 
 a1 z  a0  0.
n
Now use the property c   c  and the fact
that for any real number a, a  a, to obtain
n
n 1
an  z   an 1  z    a1  z   a0  0
n
Hence z is also a zero of (x),
which completes the proof.
n
f  z   0.
3.3 - 33
Caution It is essential that the
polynomial have only real coefficients.
For example, (x) = x – (1 + i) has 1 + i as
a zero, but the conjugate 1 – i is not a zero.
3.3 - 34
Example 5
FINDING A POLYNOMIAL FUNCTION THAT
SATISFIES GIVEN CONDITIONS
(COMPLEX ZEROS)
Find a polynomial function of least degree
having only real coefficients and zeros 3 and
2 + i.
Solution The complex number 2 – i must
also be a zero, so the polynomial has at
least three zeros, 3, 2 + i, and 2 – i. For the
polynomial to be of least degree, these must
be the only zeros. By the factor theorem
there must be three factors, x – 3, x – (2 + i),
and x – (2 – i), so
3.3 - 35
Example 5
FINDING A POLYNOMIAL FINCTION THAT
SATISFIES GIVEN CONDITIONS
(COMPLEX ZEROS)
Find a polynomial function of least degree
having only real coefficients and zeros 3 and
2 + i.
Solution
f ( x )  ( x  3)  x  (2  i ) x  (2  i )
 ( x  3)( x  2  i )( x  2  i )
 ( x  3)( x  4 x  5)
2
Remember:
i2 = –1
 x 3  7 x 2  17 x  15.
3.3 - 36
Example 5
FINDING A POLYNOMIAL FINCTION THAT
SATISFIES GIVEN CONDITIONS
(COMPLEX ZEROS)
Find a polynomial function of least degree
having only real coefficients and zeros 3 and
2 + i.
Solution Any nonzero multiple of
x3 – 7x2 + 17x – 15 also satisfies the given
conditions on zeros. The information on
zeros given in the problem is not enough to
give a specific value for the leading
coefficient.
3.3 - 37
Example 6
FINDING ALL ZEROS OF A
POLYNOMIAL FUNCTION GIVEN ONE
ZERO
Find all zeros of
(x) = x4 – 7x3 + 18x2 – 22x + 12,
given that 1 – i is a zero.
Solution Since the polynomial function has
only real coefficients and since 1 – i is a
zero, by the conjugate zeros theorem 1 + i is
also a zero. To find the remaining zeros,
first use synthetic division to divide the
original polynomial by x – (1 – i).
3.3 - 38
Example 6
FINDING ALL ZEROS OF A
POLYNOMIAL FUNCTION GIVEN ONE
ZERO
Find all zeros of
(x) = x4 – 7x3 + 18x2 – 22x + 12,
given that 1 – i is a zero.
Solution
1 i 1  7
18
 22
12
1  i  7  5i 16  6i  12
1 6i
11  5i
 6  6i
0
3.3 - 39
Example 6
FINDING ALL ZEROS OF A
POLYNOMIAL FUNCTION GIVEN ONE
ZERO
Find all zeros of
(x) = x4 – 7x3 + 18x2 – 22x + 12,
given that 1 – i is a zero.
Solution By the factor theorem, since
x = 1 – i is a zero of (x), x – (1 – i) is a
factor, and (x) can be written as
f ( x )   x  (1  i )  x  ( 6  i )x  (11  5i )x  ( 6  6i ) .
3
2
We know that x = 1 + i is also a zero of (x), so
f ( x )   x  (1  i ) x  (1  i ) q( x ),
for some polynomial q(x).
3.3 - 40
Example 6
FINDING ALL ZEROS OF A
POLYNOMIAL FUNCTION GIVEN ONE
ZERO
Find all zeros of
(x) = x4 – 7x3 + 18x2 – 22x + 12,
given that 1 – i is a zero.
Solution Thus,
x 3  ( 6  i )x 2  (11  5i )x  ( 6  6i )   x  (1  i ) q( x ).
Use synthetic division to find q(x).
1  i 1  6  i 11  5i
1  i  5  5i
1 5
6
 6  6i
6  6i
0
3.3 - 41
Example 6
FINDING ALL ZEROS OF A
POLYNOMIAL FUNCTION GIVEN ONE
ZERO
Find all zeros of
(x) = x4 – 7x3 + 18x2 – 22x + 12,
given that 1 – i is a zero.
Solution Since q(x) = x2 – 5x + 6, (x) can
be written as
f ( x )   x  (1  i ) x  (1  i )( x 2  5 x  6).
Factoring x2 – 5x + 6 as (x – 2)(x – 3), we see
that the remaining zeros are 2 and 3. The
four zeros of (x) are 1 – i, 1 + i, 2, and 3.
3.3 - 42
Descartes’ Rule of Signs
Let (x) define a polynomial function with real coefficients
and a nonzero constant term, with terms in descending
powers of x.
a. The number of positive real zeros of  either equals the
number of variations in sign occurring in the coefficients
of (x), or is less than the number of variations by a
positive even integer.
b. The number of negative real zeros of  either equals
the number of variations in sign occurring in the
coefficients of (– x), or is less than the number of
variations by a positive even integer.
3.3 - 43
APPLYING DESCARTES’ RULE OF
SIGNS
Example 7
Determine the possible number of positive
real zeros and negative real zeros of
f ( x )  x 4  6 x 3  8 x 2  2 x  1.
Solution We first consider the possible
number of positive zeros by observing that
(x) has three variations in signs:
 x 4  6 x 3  8 x 2  2x  1
1
2
3
3.3 - 44
Example 7
APPLYING DESCARTES’ RULE OF
SIGNS
Determine the possible number of positive
real zeros and negative real zeros of
f ( x )  x 4  6 x 3  8 x 2  2 x  1.
Solution Thus, by Descartes’ rule of signs,
 has either 3 or 3 – 2 = 1 positive real zeros.
For negative zeros, consider the variations in
signs for (–x):
4
3
2
f (  x )  (  x )  6(  x )  8(  x )  2(  x )  1
 x 4  6 x 3  8 x 2  2 x  1.
1
3.3 - 45
APPLYING DESCARTES’ RULE OF
SIGNS
Example 7
Determine the possible number of positive
real zeros and negative real zeros of
f ( x )  x 4  6 x 3  8 x 2  2 x  1.
Solution
 x  6 x  8 x  2 x  1.
4
3
2
1
Since there is only one variation in sign, (x)
has only 1 negative real zero.
3.3 - 46