Main content

## Algebra (all content)

### Course: Algebra (all content) > Unit 10

Lesson 8: Multiplying binomials by polynomials- Multiplying binomials by polynomials
- Multiply binomials by polynomials
- Multiplying binomials by polynomials: area model
- Multiplying binomials by polynomials challenge
- Multiplying binomials by polynomials review
- Multiplying binomials by polynomials (old)
- Multiplying binomials with radicals (old)

© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Multiplying binomials by polynomials (old)

An old video where Sal gives several examples of polynomial multiplication. Created by Sal Khan.

## Want to join the conversation?

- Would (x+2)(x-7) be x^2 - 5x + 14?(9 votes)
- You are only slightly wrong.

First term is right: x(x) = x^2

Middle term is correct, too: (-7x + 2x) = -5x

But the last term is wrong. You have to multiply 2 and (-7), so it would be -14, not +14(7 votes)

- When he gets the last part of the second problem after he finishes multiplying at3:04, is that the end of the problem? Or do you have to combine like terms and find a real answer?(4 votes)
- I expanded (x+3x^2)^5 and got x^5 +15X^6 +90x^7 +270x^8 +405x^9 +243x^10 but now I can't figure out the answer to this question: Find the coefficient of x^8 in the expansion of (1+ (x+3x^2))^5. Can someone please help me?(3 votes)
`a*x^n; a is the coefficient, n is the exponant, x is the base.`

Given:`(1 + (x + 3•x^2))^5`

Expand:`(1 + x + 3•x^2)^5`

(1 + x + 3•x^2)(1 + x + 3•x^2)(1 + x + 3•x^2)(1 + x + 3•x^2)(1 + x + 3•x^2)

(9•x^4 + 6•x^3 + 7•x^2 + 2•x + 1)(9•x^4 + 6•x^3 + 7•x^2 + 2•x + 1)(1 + x + 3•x^2)

(81•x^8 + 108•x^7 + 162•x^6 + 120•x^5 + 91•x^4 + 40•x^3 + 18•x^2 + 4•x + 1)(1 + x + 3•x^2)

(81•x^8 + 108•x^7 + 162•x^6 + 120•x^5 + 91•x^4 + 40•x^3 + 18•x^2 + 4•x + 1)(1 + x + 3•x^2)

243•x^10 + 405•x^9 + 675•x^8 + 630•x^7 + 555•x^6 + 331•x^5 + 185•x^4 + 70•x^3 + 25•x^2 + 5•x + 1

Coefficient of x^8 term = 675(5 votes)

- The method at7:29is nice but I can't make it work for something like:

(4x^2 + y^2)^2

I end up with 16x^2 + 8x^2 * 2y^2 + y ^4 but it should be 16x^2 + 8x^2 * y^2 + y ^4(2 votes) - Thank you very much, this was very precise and helped me tons!

Thank you so much again!(2 votes) - I also thought that you can not multiply a number with another number that has a variable.(2 votes)
- If after you distribute you end up with something like 6x^2- 7x^2 would you keep it as -x^2 or would you change it?(2 votes)
- Is it only 2x when you add, but when you multiply two (x times x) it's x^2?(2 votes)
- you could use the F.O.I.L method (first, outside, inside, last) for a binomial and a binomial(2 votes)
- polynomial means many terms like 4 or more terms(2 votes)

## Video transcript

In this video I'm just going
to multiply a ton of polynomials, and hopefully
that'll give you enough exposure to feel confident when
you have to multiply any for yourselves. Let's start with a fairly
simple problem. Let's say we just want to
multiply 2x times 4x minus 5. Well, we just straight
up use the distributive property here. And really, when we do all
of these polynomial multiplications, all we're
doing is the distributive property repeatedly. But let's just do the
distributive property here. This is 2x times 4x, plus
2x times negative 5. Or we could say negative
5 times 2x. So you'd say, minus
5 times 2x. All I did is distribute
the 2x. This first term is going to be
equal to-- we can multiply the coefficients. Remember, 2x times 4x is the
same thing as-- you can rearrange the order
of multiplication. This is the same thing as 2
times 4, times x times x. Which is the same thing
as 8 times x squared. Remember, x to the 1, times x
to the 1, add the exponents. I mean, you know x times
x is x squared. So this first term is going
to be 8x squared. And the second term, negative
5 times 2 is negative 10x. Not too bad. Let's do a slightly
more involved one. Let's say we had 9x to the third
power, times 3x squared, minus 2x, plus 7. So once again, we're just going
to do the distributive property here. So we're going to multiply the
9x to the third times each of these terms. So 9x to the third
times 3x squared. I'll write it out this time. In the next few, we'll
start doing it a little bit in our heads. So this is going to be 9x to
the third times 3x squared. And then we're going to have
plus-- let me write it this way-- minus 2x times 9x to the
third, and then plus 7 times 9x to the third. So sometimes I wrote the 9x to
the third first, sometimes we wrote it later because
I wanted this negative sign here. But it doesn't make a difference
on the order that you're multiplying. So this first term here
is going to be what? 9 times 3 is 27 times x to
the-- we can add the exponents, we learned that in
our exponent properties. This is x to the fifth power,
minus 2 times 9 is 18x to the-- we have x to the
1, x to the third-- x to the fourth power. Plus 7 times 9 is 63x
to the third. So we end up with this nice
little fifth degree polynomial. Now let's do one where we are
multiplying two binomials. And I'll show you what
I mean in a second. This you're going to see
very, very, very frequently in algebra. So let's say you have x minus
3, times x plus 2. And I actually want to show you
that all we're doing here is the distributive property. So let me write it like
this: times x plus 2. So let's just pretend that this
is one big number here. And it is. You know, if you had x's, this
would be some number here. So let's just distribute this
onto each of these variables. So this is going to be x minus
3, times that green x, plus x minus 3, times that green 2. All we did is distribute
the x minus 3. This is just the distributive
property. Remember, if I had a times
x plus 2, what would this be equal to? This would be equal to a
times x plus a times 2. So over here, you can see when x
minus 3 is the same thing as a, we're just distributing it. And now we would do the
distributive property again. In this case, we're distributing
the x now onto the x minus 3. We're going to distribute the
2 onto the x minus 3. You might be used to seeing the
x on the other side, but either way, we're just
multiplying it. So this is going to be--
I'll stay color coded. This is going to be x times x,
minus 3 times x, plus x times 2-- I'm going through great
pains to keep it color coded for you. I think it's helping--
minus 3 times 2. All I did is distribute the
x and distribute the 2. And soon you're going
to get used to this. We can do it in one step. You're actually multiplying
every term in this one by every term in that one, and
we'll figure out faster ways to do it in the future. But I really want to show
you the idea here. So what's this going to equal? This is going to equal
x squared. This right here is going
to be minus 3x. This is going to be plus 2x. And then this right here
is going to be minus 6. And so this is going to be x
squared minus 3 of something, plus 2 of something, that's
minus 1 of that something. Minus x, minus 6. We've multiplied those two. Now before we move on and do
another problem, I want to show you that you can kind of do
this in your head as well. You don't have to go through
all of these steps. I just want to show you really
that this is just the distributive property. The fast way of doing it, if
you had x minus 3, times x plus 2, you literally just want
to multiply every term here times each of
these terms. So you'd say, this x
times that x, so you'd have x squared. Then you'd have this x times
that 2, so plus 2x. Then you'd have this minus
3 times that x, minus 3x. And then you have the minus 3,
or the negative 3, times 2, which is negative 6. And so when you simplify, once
again you get x squared minus x minus 6. And it takes a little
bit of practice to really get used to it. Now the next thing I want to
do-- and the principal is really the exact same way-- but
I'm going to multiply a binomial times a trinomial,
which many people find daunting. But we're going to see,
if you just stay calm, it's not too bad. 3x plus 2, times 9x squared,
minus 6x plus 4. Now you could do it the exact
same way that we did the previous video. We could literally take this 3x
plus 2, distribute it onto each of these three terms,
multiply 3x plus 2 times each of these terms, and then you're
going to distribute each of those terms
into 3x plus 2. It would take a long time and in
reality, you'll never do it quite that way. But you will get the same answer
we're going to get. When you have larger
polynomials, the easiest way I can think of to multiply,
is kind of how you multiply long numbers. So we'll write it like this. 9x squared, minus 6, plus 4. And we're going to multiply
that times 3x plus 2. And what I imagine is, when you
multiply regular numbers, you have your ones' place,
your tens' place, your hundreds' place. Here, you're going to have your
constants' place, your first degree place, your second
degree place, your third degree place,
if there is one. And actually there will
be in this video. So you just have to put things
in their proper place. So let's do that. So you start here, you multiply
almost exactly like you would do traditional
multiplication. 2 times 4 is 8. It goes into the ones', or
the constants' place. 2 times negative 6x
is negative 12x. And we'll put a plus there. That was a plus 8. 2 times 9x squared is 18x
squared, so we'll put that in the x squared place. Now let's do the 3x part. I'll do that in magenta, so you
see how it's different. 3x times 4 is 12x,
positive 12x. 3x times negative 6x,
what is that? The x times the x is
x squared, so it's going to go over here. And 3 times negative
6 is negative 18. And then finally 3x times 9x
squared, the x times the x squared is x to the
third power. 3 times 9 is 27. I wrote it in the
x third place. And once again, you just
want to add the like terms. So you get 8. There's no other constant
terms, so it's just 8. Negative 12x plus 12x,
these cancel out. 18x squared minus 18x squared
cancel out, so we're just left over here with 27x
to the third. So this is equal to 27x
to the third plus 8. And we are done. And you can use this technique
to multiply a trinomial times a binomial, a trinomial times
a trinomial, or really, you know, you could have
five terms up here. A fifth degree times
a fifth degree. This will always work as long
as you keep things in their proper degree place.