5.1 Add and Subtract Polynomials

Topics covered in this section are:

1. Determine the degree of polynomials
2. Add and subtract polynomials
3. Evaluate a polynomial function for a given value
4. Add and subtract polynomial functions

5.1.1 Determine the Degree of Polynomials

We have learned that a term is a constant or the product of a constant and one or more variables. A monomial is an algebraic expression with one term. When it is of the form $ax^{m}$, where $a$ is a constant and $m$ is a whole number, it is called a monomial in one variable. Some examples of monomial in one variable are. Monomials can also have more than one variable such as and $−4a^{2}b^{3}c^{2}$.

MONOMIAL

A monomial, or two or more monomials combined by addition or subtraction, is a polynomial. Some polynomials have special names, based on the number of terms. A monomial is a polynomial with exactly one term. A binomial has exactly two terms, and a trinomial has exactly three terms. There are no special names for polynomials with more than three terms.

POLYNOMIALS

Here are some examples of polynomials.

Polynomial	$y+1$	$4a^{2}-7ab+2b^{2}$	$4x^{4}+x^{3}+8x^{2}-9x+1$
Mononomial	$14$	$8y^{2}$	$-9x^{3}y^{5}$	$-13a^{3}b^{2}c$
Binomial	$a+7b$	$4x^{2}-y^{2}$	$y^{2}-16$	$3p^{3}q-9p^{2}q$
Trinomial	$x^{2}-7x+12$	$9m^{2}+2mn-8n^{2}$	$6x^{4}-k^{3}+8k$	$z^{4}+3z^{2}-1$

Notice that every monomial, binomial, and trinomial is also a polynomial. They are just special members of the “family” of polynomials and so they have special names. We use the words monomial, binomial, and trinomialwhen referring to these special polynomials and just call all the rest polynomials.

The degree of a polynomial and the degree of its terms are determined by the exponents of the variable.

A monomial that has no variable, just a constant, is a special case. The degree of a constant is $0$.

DEGREE OF A POLYNOMIAL

Let’s see how this works by looking at several polynomials. We’ll take it step by step, starting with monomials, and then progressing to polynomials with more terms.

Let’s start by looking at a monomial. The monomial $8ab^{2}$ has two variables $a$ and $b$. To find the degree we need to find the sum of the exponents. The variable a doesn’t have an exponent written, but remember that means the exponent is $1$. The exponent of $b$ is $2$. The sum of the exponents, $1+2$, is $3$ so the degree is $3$.

Here are some additional examples.

Working with polynomials is easier when you list the terms in descending order of degrees. When a polynomial is written this way, it is said to be in standard form of a polynomial. Get in the habit of writing the term with the highest degree first.

Example 1

Determine whether each polynomial is a monomial, binomial, trinomial, or other polynomial. Then, find the degree of each polynomial.

• $7y^{2}-5y+3$
• $-2a^{4}b^{2}$
• $3x^{5}-4x^{3}-6x^{2}+x-8$
• $2y-8xy^{3}$
• $15$

5.1.2 Add and Subtract Polynomials

We have learned how to simplify expressions by combining like terms. Remember, like terms must have the same variables with the same exponent. Since monomials are terms, adding and subtracting monomials is the same as combining like terms. If the monomials are like terms, we just combine them by adding or subtracting the coefficients.

Example 2

Add or subtract:

• $25y^{2}+15y^{2}$
• $16pq^{3}-(-7pq^{3})$

Remember that like terms must have the same variables with the same exponents.

Example 3

Simplify:

• $a^{2}+7b^{2}-6a^{2}$
• $u^{2}v+5u^{2}-3v^{2}$

We can think of adding and subtracting polynomials as just adding and subtracting a series of monomials. Look for the like terms—those with the same variables and the same exponent. The Commutative Property allows us to rearrange the terms to put like terms together.

Example 4

Find the sum: $(7y^{2}-2y+9)+(4y^{2}-8y-7)$.

Be careful with the signs as you distribute while subtracting the polynomials in the next example.

Example 5

Find the difference: $(9w^{2}-7w+5)-(2w^{2}-4)$.

To subtract $a$ from $b$, we write it as $b−a$, placing the $b$ first.

Example 6

Subtract $(p^{2}+10pq-2q^{2})$ from $(p^{2}+q^{2})$.

Example 7

Find the sum: $(u^{2}-6uv+5v^{2})+(3u^{2}+2uv)$.

When we add and subtract more than two polynomials, the process is the same.

Example 8

Simplify: $(a^{3}-a^{2}b)-(ab^{2}+b^{3})+(a^{2}b+ab^{2})$.

5.1.3 Evaluate a Polynomial Function for a Given Value

A polynomial function is a function defined by a polynomial. For example, $f(x)=x^{2}+5x+6$ and $g(x)=3x−4$ are polynomial functions, because $x^{2}+5x+6$ and $3x−4$ are polynomials.

POLYNOMIAL FUNCTION

A polynomial function is a function whose range values are defined by a polynomial.

In Chapter 3, where we first introduced functions, we learned that evaluating a function means to find the value of $f(x)$ for a given value of $x$. To evaluate a polynomial function, we will substitute the given value for the variable and then simplify using the order of operations.

Example 9

For the function $f(x)=5x^{2}-8x+4$ find:

• $f(4)$
• $f(-2)$
• $f(0)$

The polynomial functions similar to the one in the next example are used in many fields to determine the height of an object at some time after it is projected into the air. The polynomial in the next function is used specifically for dropping something from $250$ ft.

Example 10

The polynomial function $h(t)=−16t^{2}+250$ gives the height of a ball $t$ seconds after it is dropped from a $250$-foot tall building. Find the height after $t=2$ seconds.

5.1.4 Add and Subtract Polynomial Functions

Just as polynomials can be added and subtracted, polynomial functions can also be added and subtracted.

ADDITION AND SUBTRACTION OF POLYNOMIAL FUNCTIONS

Example 11

For functions $f(x)=3x^{2}-5x+7$ and $g(x)=x^{2}-4x-3$, find:

• $(f+g)(x)$
• $(f+g)(3)$
• $(f-g)(x)$
• $(f-g)(-2)$