# Introduction to Multiplying Polynomials

This section covers:

# What is a Polynomial?

About the time when you learn exponents in variables, you’ll also learn how to add, subtract, multiply, and then divide (or factor) what we call polynomials.  A polynomial (meaning “many” “nomial’s”, or many terms) is basically just a collection of numbers (coefficients) multiplied by letters (variables) that can have exponents, possibly added to or subtracted from other numbers without letters (constants).  Polynomials can have fractions as coefficients, but have no variables in denominators – these are called rationals, and we will work with them later in the Rational Functions section.

Note that in order for a function to be a polynomial, it’s domain must be all real numbers!

A polynomial with one term is called a monomial; two terms is a binomial; three terms is a trinomial, and four and more terms is typically just called a polynomial.  There can be one or more variables in the polynomials, or no variables in polynomials (for example, if there is just a number, or constant).

The degree of the polynomial is the highest exponent of one of the terms (add exponents if there are more than one variable in that term); for example, the degree of  is 3 + 1 = 4.

A polynomials having a degree higher than four is typically just called a polynomial, or a polynomial of degree n, the highest degree of five is called a quintic, the highest degree of four is called a quartic, three is a cubic, two is a quadratic (you’ll hear a lot more about these!), one a linear, and zero (just a number) is a constant.  So make sure you understand how we named the polynomials below:

These are not polynomials (notice the variable in the denominator or a root of a variable):

When adding and subtracting polynomials, you just put the terms with the same variables and exponents together.   These are called “like terms”.   So  and  are like terms (adding them would be ), 4xy and yx are like terms (adding them would be 5xy), but  and  are not.

We’re actually using the distributive property (sort of backwards) when we put together like terms.  We saw this with linear functions, but it applies to other functions as well.  Notice that we have invisible “1”’s before variables with no numbers (coefficients).  For example:

Note on the last example of a polynomial above, we had two different letters (variables) in the same term; they each represent a different amount.  We’ll talk more about working with more than one variable later.

# Multiplying Polynomials: FOILING, and “Pushing Through”

Multiplying binomials (also sometimes called FOILING) is done frequently in your algebra classes.  There are two different ways to do this: the FOIL (First Outer Inner Last) method or the more generic “pushing through” method or just doing “long multiplication”.

Later, we’ll learn how to undo the multiplication of the polynomials (or factor or “UNFOIL”) when we want to turn them back into factors.

Let’s first show multiplying binomials with a multiplication boxLet’s multiply .

We split up the first binomial on the top, and the second down the left side.  (We have to watch the signs, and turn the minus 2 in the second binomial into a negative 2). Then we multiply across and down to fill in all the boxes, and then add up all the boxes:

Since we won’t want to draw a box every time we multiply binomials, we have a several methods to help us.

## FOILing

FOIL stands for First Outer Inner Last.  Just remember that we multiply the First Outer Inner and then Last terms and put plus signs between them (unless the product is negative).  Here are some examples.   Note that FOILing only works if you multiply binomials – each factor has two terms.

Note the last two examples are “special cases” that you’ll see a lot: difference of two squares, and perfect square trinomials; note the shortcuts with these cases.

## “Pushing Through” or Distributing Terms of Polynomials

Really, what we are doing when we are FOILing is using the distributive method to make sure every term (variable or number) is multiplied by every other one and then you add them all up.  We can also think of this as “pushing through” the terms to every other term.

Also called “double distributing”, this way of multiplying binomials is more popular now, since it can be used for any polynomial.

Let’s do a couple of the problems above and also see how “pushing through” can be used with polynomial products when we don’t have two binomials:

Much more multiplying and factoring polynomials will be in the Introduction to Quadratics and Graphing and Solving Polynomials sections.

Learn these rules, and practice, practice, practice!

Click on Submit (the arrow to the right of the problem) to solve this problem. You can also type in more problems, or click on the 3 dots in the upper right hand corner to drill down for example problems.

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