6

THE VOCABULARY OF
POLYNOMIAL FUNCTIONS

Terms and factors

Variables versus constants

Definition of a monomial in x

Definition of a polynomial in x

Degree of a term

Degree of a polynomial

General form of a polynomial

Domain and range

FUNCTIONS CAN BE CATEGORIZED, and the simplest type is a polynomial function.  We will define it below.  We begin with vocabulary.

1.   When numbers are added or subtracted, they are called terms.  This --

4x² + 7x − 8

-- is a sum of three terms.  (In algebra we speak of a "sum," even though a term may be subtracted.)

When numbers are multiplied, they are called factors.  This --

1.    (x + 1)(x + 2)(x + 3)

-- is a product of three factors.

2.   A variable is a symbol that takes on different values.  A value is a number.

Thus if x is a variable and we give it the value 4, then 5x + 1 has the value 21.

3.   A constant is a symbol that has a single value.  The symbols '5' and '' are numerical constants.  When we write

y = ax² + bx + c,

then  a, b, c  are undetermined constants to which we may assign a definite value; for example,

y = 5x² − 2x + 1.

We typically use the beginning letters of the alphabet to denote such constants. We use the letters x, y, z to denote variables.

4.   A monomial in x is a single term of the form  axⁿ,  where a is a
real number and n is a whole number.

The following are monomials in x:

5x³    −6.3x     2

We may say that the number 2 is a monomial in x, because 2 = 2x⁰ = 2· 1. (Lesson 21 of Algebra.)

5.   A polynomial in x is a sum of monomials in x.

Example 1.   5x³ − 4x² + 7x − 8.

The variable, in this case x, is also called the argument of the polynomial.  Here is a polynomial with argument t :

t ² −5t + 1.

When we write a polynomial, the style is to begin with the highest exponent and go to the lowest.  4, 3, 2, 1.

(For the general form of a polynomial, see Problem 6 below.)

6.   The degree of a term is the sum of the exponents of all the variables in that term.

In functions of a single variable, such as x, the degree of a term is simply the exponent.

Example 2.  The term  5x³  is of degree 3 in the variable x.

Example 3.  This term  2xy²z³ is of degree 1 + 2 + 3 = 6 in the variables x, y, and z. 

Example 4.  Here are all possible terms of the 4th degree in the variables x and y:

x⁴,  x³y,  x²y²,  xy³,  y⁴.

In each term, the sum of the exponents is 4.  As the exponent of x decreases, the exponent of y increases.

Problem 1.   Write all possible terms of the 5th degree in the variables x and y.

To see the answer, pass your mouse over the colored area.
To cover the answer again, click "Refresh" ("Reload").

x⁵,  x⁴y,  x³y²,   x²y³,  xy⁴,  y⁵.

7.   The leading term of a polynomial is the term of highest degree.

Example 5.  The leading term of this polynomial  5x³ − 4x² + 7x − 8  is 5x³.

8.   The leading coefficient of a polynomial is the coefficient of the leading term.

Example 6.  The leading coefficient of that polynomial  is 5.

9.   The degree of a polynomial is the degree of the leading term.

Example 7.  The degree of this polynomial  5x³ − 4x² + 7x − 8   is 3.

Here is a polynomial of the first degree:  x − 2.

1 is the highest exponent.

10.   The constant term of a polynomial is the term of degree 0; it is the term in which the variable does not appear.

Example 8.  The constant term of this polynomial  5x³ − 4x² + 7x − 8  is −8.

The constant term of this polynomial --

ax³ + bx² + cx + d

-- is d.

Problem 2.   Which of the following is a polynomial?  If an expression is a polynomial, name its degree, and say the variable that the polynomial is in.

a)  x³ − 2x² − 3x − 4    Polynomial of the 3rd degree in x.

b)  3y² + 2y + 1    Polynomial of the 2nd degree in y.

c)  x³ + 2 + 1     This is not a polynomial, because is not a
    whole number power. It is x^½.

d)  z + 2  Polynomial of the first degree in z.

number power.

Problem 3.    Name the degree, the leading coefficient, and the constant term.

a)  f(x) = 6x³ + 7x² − 3x + 1

3rd degree.  Leading coefficient, 6.  Constant term, 1.

b)  g(x) = −x + 2

1st degree.  Leading coefficient, −1.  Constant term, 2.

c)  h(x) = 4x⁵

5th degree.  Leading coefficient, 4.  Constant term, 0.

d)  f(h) = h² − 7h − 5

2nd degree.  Leading coefficient, 1.  Constant term, −5.

Example 9.   Name the degree, the leading coefficient, and the constant term of  (5x + 1)(3x − 1)(2x + 5)³.

11.   The general form of a polynomial shows the terms of all possible degree.  Here, for example, is the general form of a polynomial of the third degree:

ax³ + bx² + cx + d

Notice that there are four constants: a, b, c, d.

In the general form, the number of constants, because of the term of degree 0, is always one more than the degree of the polynomial.

Now, to indicate a polynomial of the 50th degree, we cannot indicate the constants by resorting to different letters.  Instead, we use sub-script notation.  We use one letter, such as a, and indicate different constants by means of sub-scripts.  Thus, a₁ ("a sub-1") will be one constant.  a₂ ("a sub-2") will be another.  And so on.  Here, then, is the general form of a polynomial of the 50th degree:

a₅₀x⁵⁰ + a₄₉x⁴⁹ + . . . + a₂x² + a₁x + a₀

The constant a_k -- for each sub-script k  (k = 0, 1, 2, . . . , 50) -- is the coefficient of x^k.

Notice that there are 51 constants.  The constant term a₀ is the 51st.

Problem 5.

a)  Using subscript notation, write the general form of a polynomial of
a)  the fifth degree in x.

a₅x⁵ + a₄x⁴ + a₃x³ + a₂x² + a₁x + a₀

b)  In that general form, how many constants are there?  6

c)  Name the six constants of this fifth degree polynomial:  x⁵ + 6x² − x.

a₅ = 1.  a₄ = 0.  a₃ = 0.  a₂ = 6.  a₁ = −1.  a₀ = 0.

Problem 6.

a)  Indicate the general form of a polynomial in x of degree n.

a_nxⁿ + a_n−1xⁿ⁻¹ + . . . + a₁x +a₀

n is a whole number, the a's are real numbers, and a_n0.

b)  A polynomial of degree n has how many constants?  n + 1

12.  A polynomial function has the form

y = A polynomial

A polynomial function of the first degree, such as  y = 2x + 1,  is called a linear function; while a polynomial function of the second degree, such as  y = x² + 3x − 2,  is called a quadratic.

The natural domain of any polynomial function is

− < x < .

x may take on any real value.  Consider the graphs of y = x² , and y = x³.

Problem 7.   Let f(x) be the function with the given, restricted domain.  Describe its range.

(If you are not viewing this page with Internet Explorer 6, then your browser may not be able to display the symbol ≤, "is less than or equal to;" or ≥, "is greater than or equal to.")

a)  f(x) = x²,   −3 ≤ x ≤ 3

0 ≤ y ≤ 9.  y goes from a low of 0 (at x = 0) to a high of 9 (at both −3 and 3).

b)  f(x) = x³,    −3 ≤ x ≤ 3

−27 ≤ y ≤ 27.  y goes from a low of −27 (at x = −3) to a high of 27 (at x = 3).

c)  f(x) = x⁴,   −2 ≤ x ≤ 1

0 ≤ y ≤ 16.   y goes from a low of 0, at x = 0, to a high of 16, at x = −2.  x⁴ is very much like x².  The exponent is even.

d)  f(x) = x⁵,   −2 ≤ x ≤ 1

−32 ≤ y ≤ 1.   y goes from a low of −32, at x = −2, to a high of 1, at x = 1.  x⁵ is very much like x³.  The exponent is odd.

In the following Topics we will focus on the graphs of these polynomial functions.

Next Topic:  The roots, or zeros, of a polynomial

Table of Contents | Home

Please make a donation to keep TheMathPage online.
Even $1 will help.

Copyright © 2012 Lawrence Spector

Questions or comments?

E-mail:  themathpage@nyc.rr.com