Main content

# Multiplying binomials intro

Sal expresses (x-4)(x+7) as the standard trinomial x²+3x-28 and discusses how the general product (x+a)(x+b) can be written as x²+(a+b)x+a*b.

## Want to join the conversation?

- What are your guys reasons for doing khan?(12 votes)
- dopamine hit from getting all questions correct(49 votes)

- At0:22he said "standard Quadratic form". Previously he spoke about standard from and just said you order in from greatest degree to lowest degree and didn't mention the Quadratic part, is standard Quadratic form more specific?? And what does the 'Quadratic' part explain?(11 votes)
- He's just specifying the type of equation he's working with.
**Extra**info: There are four specific ways to reference the degree (the greatest exponent) of an expression:

Linear: degree of 1; (x+2),(x)

Quadratic: degree of 2; (x^2-5x+2),(x^2+3),(x^2+8x),(x^2)

Cubic: degree of 3; (x^3+x^2+x+1)

Quartic: degree of 4; (x^4)

It doesn't matter how many terms are in an expression when you're trying to determine its degree.**Information**related to the kind of examples given: The number of terms in an expression has three specific words, and one general word.

Monomial: Only one term

Binomial: Two terms

Trinomial: Three terms

Polynomial (general): More than three terms(39 votes)

- Why plus seven at the start?(11 votes)
- I think the idea was that you take the two terms, here being x and positive 7 and distributing each term to (x-4) and once you get the binomial products, you add them together to get the answer of x^2+3x-28. Another method to multiply binomials is FOIL or First Outside Inside Last. Both of these methods will give you the same answers but FOIL is typically faster. Hope this helps!(11 votes)

- What's the "FOIL" method that everyone in the comments is talking about?(1 vote)
- FOIL stands for
**First Outside Inside Last**. It's a mnemonic for multiplying two binomials, and here's how it works!

Let's say we have the expression (2+4)(7-5).

We start by multiplying the**first**terms:

2*7

Then we add the product of the**inside**(or closest to the middle) terms:

2*7 + 4*7

Next we add the product of the**outside**terms

2*7 + 4*7+ 2*-5

And finally, we add the product of the**last**terms.

2*7 + 4*7 + 2*-5 + 4*-5

Now we solve it. It's not necessary, but it is a little easier to solve if we put parentheses in:

(2*7)+(4*7)+(2*-5)+(4*-5)

Which becomes

14 + 28 + (-10) + (-20)

42-30

=12

And that's FOIL!

Edit: ACK! I messed up the order of the instructions. (I gave you FIOL instead of FOIL!) But it doesn’t really matter which order you do it in as long as you keep track of your terms. So if you want to do it the official way (for example, if you have to show your work) just switch Inside and Outside in my instructions. Sorrrrrryyyyyyyyyyyy 😅🙇🏻♀️(26 votes)

- The general product can be written as shown above because he is basically taking steps instead of doing it at once .(8 votes)
- Doesn't FOIL seems easier and it takes fewer steps? To me it does.(4 votes)
- Foil is often easier for multiplying two binomials, but you need other methods later on to do trinomials by binomials or trinomials by trinomials.(4 votes)

- in an exam do i have to write every step?

cuz this is how i do it

(x-4)(x+7)

x^2+7x-4x-28(1 vote)- Most teachers ask you to show your work so if you make an error they can help you understand it and correct it. But, you need to ask your teacher what they require.(10 votes)

- What's the "FOIL" method?(2 votes)
- Why is this put under High school

math if it is put under Algebra 1?(4 votes)- KhanAcademy has lesson by topic / course like Algebra 1. And it has lessons aligned by grade level. They reuse videos where the content is in common.(3 votes)

- So much math everywhere(5 votes)

## Video transcript

- [Voiceover] Let's see
if we can figure out the product of x minus
four and x plus seven. And we want to write that product
in standard quadratic form which is just a fancy way of saying a form where you have some coefficient
on the second degree term, a x squared plus some coefficient b on the first degree term
plus the constant term. So this right over here would
be standard quadratic form. So that's the form that we
want to express this product in and encourage you to pause the video and try to work through it on your own. Alright, now let's work through this. And the key when we're multiplying
two binomials like this, or actually when you're
multiplying any polynomials, is just to remember the
distributive property that we all by this point know quite well. So what we could view this is as is we could distribute this x minus four, this entire expression
over the x and the seven. So we could say that
this is the same thing as x minus four times x plus x minus four times seven. So let's write that. So x minus four times x,
or we could write this as x times x minus four. That's distributing, or multiplying
the x minus four times x that's right there. Plus seven times x minus four. Times x minus four. Notice all we did is
distribute the x minus four. We took this whole thing
and we multiplied it by each term over here. We multiplied x by x minus four and we multiplied seven by x minus four. Now, we see that we have these, I guess you can call
them two seperate terms. And to simplify each of them,
or to multiply them out, we just have to distribute. In this first we're going to
have to distribute this blue x. And over here we have to
distribute this blue seven. So let's do that. So here we can say x times
x is going to be x squared. X times, we have a negative here, so we can say negative four is
going to be negative four x. And just like, that we get
x squared minus four x. And then over here we have seven times x so that's going to be plus seven x. And then we have seven
times the negative four which is negative 28. And we are almost done. We can simplify it a little bit more. We have two first degree terms here. If I have negative four xs
and to that I add seven xs, what is that going to be? Well those two terms together, these two terms together are going to be negative four plus seven xs. Negative four plus, plus seven. Negative four plus seven xs. So all I'm doing here,
I'm making it very clear that I'm adding these two coefficients, and then we have all of the other terms. We have the x squared. X squared plus this and then we have, and then we have the minus, and then we have the minus 28. And we're at the home stretch! This would simply to x squared. Now negative four plus seven is three, so this is going to be plus three x. That's what these two middle
terms simplify to, to three x. And then we have minus 28. Minus 28. And just like that, we are done! And a fun thing to think about,
since it's in the same form. If we were to compare a is one, b is three, and c is -28, but it's interesting here
to look at the pattern when we multiplied these two binomials. Especially these two binomials
where the coefficient on the x term was a one. Notice we have x times x, that what actually forms the
x squared term over here. We have negative four, let
me do this in a new color. We have negative four times,
that's not a new color. We have, we have negative four times seven, which is going to be negative 28. And then how did we get this middle term? How did we get this three x? Well, you had the negative
four x plus the seven x. Or the negative four
plus the seven times x. You had the negative four plus the seven, plus the seven times x. So I hope you see a little
bit of a pattern here. If you're multiplying two binomials where the coefficients on
the x term are both one. It's going to be x squared. And then the last term, the constant term, is going to be the product
of these two constants. Negative four and seven. And then the first degree
term right over here, it's coefficient is going to be the sum of these two constants,
negative four and seven. Now this might, you could do this pattern if you practice it. It's just something that will help you multiply binomials a little bit faster. But it's super important that you realize where this came from. This came from nothing more than applying the distributive property twice.