If
âŽ
p(x) is a polynomial, the solutions to the equation
âŽ
p(x) = 0 are called the zeros of the
polynomial. Sometimes the zeros of a polynomial can be determined by factoring or by using the
Quadratic Formula, but frequently the zeros must be approximated. The real zeros of a polynomial
p(x) are the x-intercepts of the graph of
âŽ
y = p(x).
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The Factor Theorem: If
âŽ
(x â k) is a factor of a polynomial, then
âŽ
x = k is a zero of the polynomial.
Conversely, if
âŽ
x = k is a zero of a polynomial, then
âŽ
(x â k) is a factor of the polynomial.
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Example 1: Find the zeros and x-intercepts of the graph of
âŽ
p(x) =x
4â5x
2 + 4.
âŽ
x
4â5x
2 + 4 = 0
(x
2 â 4)(x
2 â1) = 0
(x + 2)(x â 2)(x +1)(x â1) = 0
x + 2 = 0 or x â 2 = 0 or x +1= 0 or x â1= 0
x = â2 or x = 2 or x = â1 or x =1
So the zeros are â2, 2, â1, and 1 and the x-intercepts are (â2,0), (2,0), (â1,0), and (1,0).
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The number of times a factor occurs in a polynomial is called the multiplicity of the factor. The
corresponding zero is said to have the same multiplicity. For example, if the factor
âŽ
(x â 3) occurs to
the fifth power in a polynomial, then
âŽ
(x â 3) is said to be a factor of multiplicity 5 and the
corresponding zero, x=3, is said to have multiplicity 5. A factor or zero with multiplicity two is
sometimes said to be a double factor or a double zero. Similarly, a factor or zero with multiplicity
three is sometimes said to be a triple factor or a triple zero.
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Example 2: Determine the equation, in factored form, of a polynomial
âŽ
p(x) that has 5 as double
zero, â2 as a zero with multiplicity 1, and 0 as a zero with multiplicity 4.
âŽ
p(x) = (x â 5)
2(x + 2)x
4
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Example 3: Give the zeros and their multiplicities for
âŽ
p(x) = â12x
4 + 36x3 â 21x
2.
âŽ
â12x
4 + 36x3 â 21x
2 = 0
â3x
2(4x
2 â12x + 7) = 0
â3x
2 = 0 or 4x
2 â12x + 7 = 0
x
2 = 0 or x = â(â12)Âą (â12)
2â4(4)(7)
2(4)
x = 0 or x = 12Âą 144â112
8 = 12Âą 32
8 = 12Âą4 2
8 = 12
8 Âą 4 2
8 = 3
2 Âą 2
2
So 0 is a zero with multiplicity 2,
âŽ
x = 3
2 â 2
2 is a zero with multiplicity 1, and
âŽ
x = 3
2 + 2
2 is a zero
with multiplicity 1.
(Thomason - Fall 2008)
Because the graph of a polynomial is connected, if the polynomial is positive at one value of x and
negative at another value of x, then there must be a zero of the polynomial between those two values
of x.
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Example 4: Show that
âŽ
p(x) = 2x3 â 5x
2 + 4 x â 7 must have a zero between
âŽ
x =1 and
âŽ
x = 2.
âŽ
p(1) = 2(1)
3 â 5(1)
2 + 4(1) â 7 = 2(1) â 5(1) + 4 â 7 = 2 â 5 + 4 â 7 = â6
and
âŽ
p(2) = 2(2)3 â 5(2)
2 + 4(2) â 7 = 2(8) â 5(2) + 8 â 7 =16 â10 + 8 â 7 = 7.
Because
âŽ
p(1) is negative and
âŽ
p(2) is positive and because the graph of
âŽ
p(x) is connected,
âŽ
p(x)
must equal 0 for a value of x between 1 and 2.
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If a factor of a polynomial occurs to an odd power, then the graph of the polynomial actually goes
across the x-axis at the corresponding x-intercept. An x-intercept of this type is sometimes called an
odd x-intercept. If a factor of a polynomial occurs to an even power, then the graph of the
polynomial "bounces" against the x-axis at the corresponding x-intercept, but not does not go across
the x-axis there. An x-intercept of this type is sometimes called an even x-intercept.
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Example 5: Use a graphing calculator or a computer program to graph
âŽ
y = 0.01x
2(x + 2)3(x â 2)(x â 4)
4 .
x
y
â2 2 4
5
Because the factors
âŽ
(x + 2) and
âŽ
(x â 2) appear to odd
powers, the graph crosses the x-axis at
âŽ
x = â2
and
âŽ
x = 2.
Because the factors x and
âŽ
(x â 4) appear to even
powers, the graph bounces against the x-axis at
âŽ
x = 0
and
âŽ
x = 4.
Note that if the factors of the polynomial were
multipled out, the leading term would be
âŽ
0.01x10.
This accounts for the fact that both tails of the graph
go up; in other words, as
âŽ
x â ââ,
âŽ
y
Step-by-step explanation: