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9.1: Addition and Subtraction of Polynomials

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Learning Objectives

  • Write a polynomial expression in standard form.
  • Classify polynomial expression by degree.
  • Add and subtract polynomials.
  • Solve problems using addition and subtraction of polynomials.

Introduction

So far we’ve seen functions described by straight lines (linear functions) and functions where the variable appeared in the exponent (exponential functions). In this section we’ll introduce polynomial functions. A polynomial is made up of different terms that contain positive integer powers of the variables. Here is an example of a polynomial:

4x^3+2x^2-3x+1

Each part of the polynomial that is added or subtracted is called a term of the polynomial. The example above is a polynomial with four terms.

The numbers appearing in each term in front of the variable are called the coefficients. The number appearing all by itself without a variable is called a constant.

In this case the coefficient of x^3 is 4, the coefficient of x^2 is 2, the coefficient of x is -3 and the constant is 1.

Degrees of Polynomials and Standard Form

Each term in the polynomial has a different degree. The degree of the term is the power of the variable in that term.

& 4x^3 && \text{has degree} \ 3 \ \text{and is called a cubic term or} \ 3^{rd} \ \text{order term}.\\& 2x^2 && \text{has degree} \ 2 \ \text{and is called a quadratic term or} \ 2^{nd} \ \text{order term}.\\& -3x && \text{has degree} \ 1 \ \text{and is called a linear term or} \ 1^{st} \ \text{order term}.\\& 1 && \text{has degree} \ 0 \ \text{and is called the constant}.

By definition, the degree of the polynomial is the same as the degree of the term with the highest degree. This example is a polynomial of degree 3, which is also called a “cubic” polynomial. (Why do you think it is called a cubic?).

Polynomials can have more than one variable. Here is another example of a polynomial:

t^4-6s^3t^2-12st+4s^4-5

This is a polynomial because all the exponents on the variables are positive integers. This polynomial has five terms. Let’s look at each term more closely.

Note: The degree of a term is the sum of the powers on each variable in the term. In other words, the degree of each term is the number of variables that are multiplied together in that term, whether those variables are the same or different.

& t^4 && \text{has a degree of} \ 4, \ \text{so it’s a} \ 4^{th} \ \text{order term}\\& -6s^3t^2 && \text{has a degree of} \ 5, \ \text{so it’s a} \ 5^{th} \ \text{order term}.\\& -12st && \text{has a degree of} \ 2, \ \text{so it’s a} \ 2^{nd} \ \text{order term}.\\& 4s^4 && \text{has a degree of} \ 4, \ \text{so it’s a} \ 4^{th} \ \text{order term}.\\& -5 && \text{is a constant, so its degree is} \ 0.

Since the highest degree of a term in this polynomial is 5, then this is polynomial of degree 5^{th} or a 5^{th} order polynomial.

A polynomial that has only one term has a special name. It is called a monomial (mono means one). A monomial can be a constant, a variable, or a product of a constant and one or more variables. You can see that each term in a polynomial is a monomial, so a polynomial is just the sum of several monomials. Here are some examples of monomials:

b^2 \qquad -2ab^2 \qquad 8 \qquad \frac{1}{4}x^4 \qquad -29xy

Example 1

For the following polynomials, identify the coefficient of each term, the constant, the degree of each term and the degree of the polynomial.

a) x^5-3x^3+4x^2-5x+7

b) x^4-3x^3y^2+8x-12

Solution

a) x^5-3x^3+4x^2-5x+7

The coefficients of each term in order are 1, -3, 4, and -5 and the constant is 7.

The degrees of each term are 5, 3, 2, 1, and 0. Therefore the degree of the polynomial is 5.

b) x^4-3x^3y^2+8x-12

The coefficients of each term in order are 1, -3, and 8 and the constant is -12.

The degrees of each term are 4, 5, 1, and 0. Therefore the degree of the polynomial is 5.

Example 2

Identify the following expressions as polynomials or non-polynomials.

a) 5x^5-2x

b) 3x^2-2x^{-2}

c) x\sqrt{x}-1

d) \frac{5}{x^3+1}

e) 4x^\frac{1}{3}

f) 4xy^2-2x^2y-3+y^3-3x^3

Solution

a) This is a polynomial.

b) This is not a polynomial because it has a negative exponent.

c) This is not a polynomial because it has a radical.

d) This is not a polynomial because the power of x appears in the denominator of a fraction (and there is no way to rewrite it so that it does not).

e) This is not a polynomial because it has a fractional exponent.

f) This is a polynomial.

Often, we arrange the terms in a polynomial in order of decreasing power. This is called standard form.

The following polynomials are in standard form:

4x^4-3x^3+2x^2-x+1

a^4b^3-2a^3b^3+3a^4b-5ab^2+2

The first term of a polynomial in standard form is called the leading term, and the coefficient of the leading term is called the leading coefficient.

The first polynomial above has the leading term 4x^4, and the leading coefficient is 4.

The second polynomial above has the leading term a^4b^3, and the leading coefficient is 1.

Example 3

Rearrange the terms in the following polynomials so that they are in standard form. Indicate the leading term and leading coefficient of each polynomial.

a) 7-3x^3+4x

b) ab-a^3+2b

c) -4b+4+b^2

Solution

a) 7-3x^3+4x becomes -3x^3+4x+7. Leading term is -3x^3; leading coefficient is -3.

b) ab-a^3+2b becomes -a^3+ab+2b. Leading term is -a^3; leading coefficient is -1.

c) -4b+4+b^2 becomes b^2-4b+4. Leading term is b^2; leading coefficient is 1.

Simplifying Polynomials

A polynomial is simplified if it has no terms that are alike. Like terms are terms in the polynomial that have the same variable(s) with the same exponents, whether they have the same or different coefficients.

For example, 2x^2y and 5x^2y are like terms, but 6x^2y and 6xy^2 are not like terms.

When a polynomial has like terms, we can simplify it by combining those terms.

& x^2+\underline{6xy} - \underline{4xy} + y^2\\& \qquad \nearrow \qquad \nwarrow\\& \qquad \text{Like terms}

We can simplify this polynomial by combining the like terms 6xy and -4xy into (6-4)xy, or 2xy. The new polynomial is x^2+2xy+y^2.

Example 4

Simplify the following polynomials by collecting like terms and combining them.

a) 2x -4x^2+6+x^2-4+4x

b) a^3b^3-5ab^4+2a^3b-a^3b^3+3ab^4-a^2b

Solution

a) Rearrange the terms so that like terms are grouped together: (-4x^2+x^2)+(2x+4x)+(6-4)

Combine each set of like terms: -3x^2+6x+2

b) Rearrange the terms so that like terms are grouped together: (a^3b^3-a^3b^3)+(-5ab^4+3ab^4)+2a^3b-a^2b

Combine each set of like terms: 0-2ab^4+2a^3b-a^2b=-2ab^4+2a^3b-a^2b

Adding and Subtracting Polynomials

To add two or more polynomials, write their sum and then simplify by combining like terms.

Example 5

Add and simplify the resulting polynomials.

a) Add 3x^2-4x+7 and 2x^3-4x^2-6x+5

b) Add x^2-2xy+y^2 and 2y^2-3x^2 and 10xy+y^3

Solution

a) & (3x^2-4x+7)+(2x^3-4x^2-6x+5)\\\text{Group like terms:} & = 2x^3+(3x^2-4x^2)+(-4x-6x)+(7+5)\\\text{Simplify:} & =  2x^3-x^2-10x+12

b) & (x^2-2xy+y^2)+(2y^2-3x^2)+(10xy+y^3)\\\text{Group like terms:} & = (x^2-3x^2)+(y^2+2y^2)+(-2xy+10xy)+y^3\\\text{Simplify:} & = -2x^2+3y^2+8xy+y^3

To subtract one polynomial from another, add the opposite of each term of the polynomial you are subtracting.

Example 6

a) Subtract x^3-3x^2+8x+12 from 4x^2+5x-9

b) Subtract 5b^2-2a^2 from 4a^2-8ab-9b^2

Solution

a) (4x^2+5x-9)-(x^3-3x^2+8x+12) & = (4x^2+5x-9)+(-x^3+3x^2-8x-12)\\\text{Group like terms:} & = -x^3+(4x^2+3x^2)+(5x-8x)+(-9-12)\\\text{Simplify:} & = -x^3+7x^2-3x-21

b) (4a^2-8ab-9b^2)-(5b^2-2a^2) & = (4a^2-8ab-9b^2)+(-5b^2+2a^2)\\\text{Group like terms:} & = (4a^2+2a^2)+(-9b^2-5b^2)-8ab\\\text{Simplify:} & = 6a^2-14b^2-8ab

Note: An easy way to check your work after adding or subtracting polynomials is to substitute a convenient value in for the variable, and check that your answer and the problem both give the same value. For example, in part (b) above, if we let a=2 and b=3, then we can check as follows:

& \text{Given} && \text{Solution}\\& (4a^2-8ab-9b^2)-(5b^2-2a^2) && 6a^2-14b^2-8ab\\& (4(2)^2-8(2)(3)-9(3)^2)-(5(3)^2-2(2)^2) && 6(2)^2-14(3)^2-8(2)(3)\\& (4(4)-8(2)(3)-9(9))-(5(9)-2(4)) && 6(4)-14(9)-8(2)(3)\\& (-113)-37 && 24-126-48\\& -150 && -150

Since both expressions evaluate to the same number when we substitute in arbitrary values for the variables, we can be reasonably sure that our answer is correct.

Note: When you use this method, do not choose 0 or 1 for checking since these can lead to common problems.

Problem Solving Using Addition or Subtraction of Polynomials

One way we can use polynomials is to find the area of a geometric figure.

Example 7

Write a polynomial that represents the area of each figure shown.

a)

b)

c)

d)

Solution

a) This shape is formed by two squares and two rectangles.

\text{The blue square has area} \ y \times y & = y^2.\\\text{The yellow square has area} \ x \times x & = x^2.\\\text{The pink rectangles each have area} \ x \times y & = xy.

To find the total area of the figure we add all the separate areas:

Total \ area &= y^2 + x^2 + xy + xy\\& = y^2 + x^2 + 2xy

b) This shape is formed by two squares and one rectangle.

\text{The yellow squares each have area} \ a \times a & = a^2.\\\text{The orange rectangle has area} \ 2a \times b & = 2ab.

To find the total area of the figure we add all the separate areas:

Total \ area & = a^2 + a^2 + 2ab\\& = 2a^2 + 2ab

c) To find the area of the green region we find the area of the big square and subtract the area of the little square.

\text{The big square has area}: y \times y & = y^2.\\\text{The little square has area}: x \times x & = x^2.\\Area \ of \ the \ green \ region & = y^2 - x^2

d) To find the area of the figure we can find the area of the big rectangle and add the areas of the pink squares.

\text{The pink squares each have area} \ a \times a & = a^2.\\\text{The blue rectangle has area} \ 3a \times a & = 3a^2.

To find the total area of the figure we add all the separate areas:

Total \ area = a^2 + a^2 + a^2 + 3a^2 = 6a^2

Another way to find this area is to find the area of the big square and subtract the areas of the three yellow squares:

\text{The big square has area} \ 3a \times 3a & = 9a^2.\\\text{The yellow squares each have area} \ a \times a & = a^2.

To find the total area of the figure we subtract:

Area & = 9a^2 - (a^2 + a^2 + a^2)\\& = 9a^2 - 3a^2 \\& = 6a^2

Further Practice

For more practice adding and subtracting polynomials, try playing the Battleship game at http://www.quia.com/ba/28820.html. (The problems get harder as you play; watch out for trick questions!)

Review Questions

Indicate whether each expression is a polynomial.

  1. x^2+3x^{\frac{1}{2}}
  2. \frac{1}{3}x^2y-9y^2
  3. 3x^{-3}
  4. \frac{2}{3}t^2-\frac{1}{t^2}
  5. \sqrt{x}-2x
  6. \left ( x^\frac{3}{2} \right )^2

Express each polynomial in standard form. Give the degree of each polynomial.

  1. 3-2x
  2. 8-4x+3x^3
  3. -5+2x-5x^2+8x^3
  4. x^2-9x^4+12
  5. 5x+2x^2-3x

Add and simplify.

  1. (x+8)+(-3x-5)
  2. (-2x^2+4x-12)+(7x+x^2)
  3. (2a^2b-2a+9)+(5a^2b-4b+5)
  4. (6.9a^2-2.3b^2+2ab)+(3.1a-2.5b^2+b)
  5. \left ( \frac{3}{5}x^2-\frac{1}{4}x+4 \right )+ \left ( \frac{1}{10}x^2 + \frac{1}{2}x-2\frac{1}{5} \right )

Subtract and simplify.

  1. (-t+5t^2)-(5t^2+2t-9)
  2. (-y^2+4y-5)-(5y^2+2y+7)
  3. (-5m^2-m)-(3m^2+4m-5)
  4. (2a^2b-3ab^2+5a^2b^2)-(2a^2b^2+4a^2b-5b^2)
  5. (3.5x^2y-6xy+4x)-(1.2x^2y-xy+2y-3)

Find the area of the following figures.

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