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## Get ready for Algebra 1

### Course: Get ready for Algebra 1 > Unit 1

Lesson 7: Combining like terms# Combining like terms with negative coefficients

CCSS.Math:

This example of combining like terms in an expression get a little hairy. Pay attention. Created by Sal Khan.

## Want to join the conversation?

- So x's cannot be combined with xy's or with x^2's, and y's cannot be combined with xy's or with y^2's, right? Doesn't this leave your equation containing a lot of terms still? Wouldn't it be a lot simpler if we could reduce the number of terms more?(275 votes)
- The way I think about it, if x = Turkey and y = farm, then 2 x (Turkeys) are not 3y (Farms) and close but not quite 4xy (Turkey Farms) xy. I just went back to the earlier videos and substituted variables for objects to make myself understand it.(102 votes)

- How do you recognize the difference between the 'minus' sign and the 'negative' sign.

I'm so confused(67 votes)- Think of them as being the same thing, it is one or the other based on where it is in the equation. The negative sign is used at the beginning of something new, so it could be primarily in three places -3(-4 + x)/-6. A negative is beginning of an expression, the beginning of a parentheses, and the beginning of the denominator. A minus is between things such as 3(2x - 5) - 5(4x - 2) . In this case, it is between an x and a constant and between two expressions. In this case, if you try to distribute -5(4x-2) independently, the minus now is at the beginning and so it is considered a negative sign rather than the minus it was in the full expression. Does this help?(83 votes)

- I have been having trouble understand the -5+6=? I don't understand last video he had two numbers with - signs and he added them and kept the sign why? What am I missing maybe video that could help it seems so simple but I have no clue! Any feed back will be welcomed.(14 votes)
- I think the easiest way to understand this is via the number line.

When we add we move to the right along the number line, and when we subtract we move to the left.

Starting at 0 (zero) we move 5 steps to the left and from there we move 6 steps to the right.`−7 −6 −5 −4 −3 −2 −1 0 1 2 3 4 5 6 7`

`−−+−−+−−+−−+−−+−−+−−+−−+−−+−−+−−+−−+−−+−−+−−+−−`

`<−−−−( −5 )−−−−−`

`−−−−−−( +6 )−−−−−−>`

As you can see, this lands us at 1.

So, −5 + 6 = 1

– – –

Adding two negative numbers, for example (−3) + (−4), is the same as (from 0) moving 3 steps to the left and then another 4 steps to the left, in total 7 steps to the left, which lands us on (−7),

and we write −3 − 4 = −(3 + 4) = −7

– – –

Sal explains it more thoroughly here:

https://www.khanacademy.org/math/arithmetic/arith-review-negative-numbers/arith-review-sub-neg-intro/v/adding-and-subtracting-negative-number-examples(65 votes)

- at1:29why did he leave out the y2 shouldn't that count as a y to?(13 votes)
- if y=3, then y2 would be 9, so he can't add them together(7 votes)

- can you guys pls upvote and comment i need achievements(16 votes)
- bros grinding khan academy(6 votes)

- At 0.21, what is the difference between x^2 and 2x?(3 votes)
- Exponents are shorthand for repetitive multiplication. So
`x^2`

=`x * x`

Multiplication is shorthand for repetitive addition:`2x`

=`x + x`

Hope this helps.(21 votes)

- this is confusing.

why don't you just give us the value of x and y so then I can solve it?(9 votes)- You won't always know the values of the variables. Part of learning algebra is that you need to learn the rules for how to manipulate algebraic expression that contain variables. This video is trying to teach one of those techniques. It is a basic skill you need to understand before you start solving algebria equations where you will actual discover for yourself what value the variable can be.(8 votes)

- Sal can you PLEASE tell me what we are supposed to use our Energy Points for?!(9 votes)
- the higher the points you have the more avatars you unlock thats all i know(5 votes)

- Is there any reason why this, [X+2*(X)] + [Y-1*(Y)] , wouldn't work as the answer?

My logic here is that X^2 and Y^2 are equal to X*(X) and Y*(Y) meaning they could be combined with -1*(Y) and 2*(X).(7 votes)- You can write it as x(x+2)+y(y-1), but this is not combining terms, it is factoring out a common factor. x+2x=3x is different from x(x+2)=x*x+x*2=x^2+2x though, so it wouldn't work as the answer.(6 votes)

- HELP?

On some examples it shows that we move the numbers on the other side of the equal = sign... I get that once thats done the numbers either become a negative or positive but... how do we know when to use it. There's some questions I can solve using the same method in this video but then there's this eg:

-3n - 7 + (-6n) + 1

If I were to use the method above would it be written as so?

-3n - (-6n) -7 + 1 ?

In this equation they used

-3n - 7 + (-6n) + 1

= (3+6)n -7 +1

I don't get it? which method do we use? if both... how are we supposed to know which one is right? or when to use either one? Please.(7 votes)- First, you need to recognize an equation vs an expression.

An expression contains**no**equals sign to start with. All the examples in this video (and your example in your question) are expressions. An equation is made up of 2 expressions separated by an equals sign.

We only change the signs to move terms across an equals sign. This requires that you have an equation. When you changed: -3n - 7 + (-6n) + 1 into -3n - (-6n) -7 + 1, you moved the "-(-6n)" to the other side of the -7. You did**not**more it across and equals sign. So you do not change the sign.

Hope this helps.(6 votes)

## Video transcript

Now we have a very, very,
very hairy expression. And once again, I'm going to
see if you can simplify this. And I'll give you
little time to do it. So this one is even crazier than
the last few we've looked at. We've got y's and
xy's, and x squared and x's, well more just xy's and
y squared and on and on and on. And there will be a temptation,
because you see a y here and a y here to
say, oh, maybe I can add this negative
3y plus this 4xy somehow since I see a y and a y. But the important
thing to realize here is that a y is
different than an xy. Think about it
they were numbers. If y was 3 and an x was a
2, then a y would be a 3 while an xy would have been a 6. And a y is very different
than a y squared. Once again, if the why it
took on the value 3, then the y squared would
be the value 9. So even though you see
the same letter here, they aren't the same-- I
guess you cannot add these two or subtract these two terms. A y is different
than a y squared, is different than an xy. Now with that said, let's
see if there is anything that we can simplify. So first, let's think
about the y terms. So you have a negative 3y there. Do we have any more y term? Yes, we do. We have this 2y
right over there. So I'll just write it
out-- I'll just reorder it. So we have negative 3y plus 2y. Now, let's think
about-- and I'm just going in an arbitrary order,
but since our next term is an xy term-- let's think about
all of the xy terms. So we have plus 4xy
right over here. So let me just write
it down-- I'm just reordering the whole
expression-- plus 4xy. And then I have minus
4xy right over here. Then let's go to
the x squared terms. I have negative 2 times x
squared, or minus 2x squared. So let's look at this. So I have minus 2x squared. Do I have any other x squared? Yes, I do. I have this 3x squared
right over there. So plus 3x squared. And then let's see, I have
an x term right over here, and that actually looks
like the only x term. So that's plus 2x. And then I only
have one y squared term-- I'll circle that in
orange-- so plus y squared. So all I have done is I've
reordered the statement and I've color coded it based
on the type of term we have. And now it should be
a little bit simpler. So let's try it out. If I have negative
3 of something plus 2 of that something,
what do I have? Or another way to say it,
if I have two of something and I subtract 3 of that,
what am I left with? Well, I'm left with negative
1 of that something. So I could write negative 1y, or
I could just write negative y. And another way you
could think about it, but I like to think about
it intuitively more, is what's the coefficient here? It is negative 3. What's the coefficient here? It's 2. Where obviously
both are dealing-- they're both y
terms, not xy terms, not y squared terms, just y. And so negative 3
plus 2 is negative 1, or negative 1y is the
same thing as negative y. So those simplify to
this right over here. Now let's look at the xy terms. If I have 4 of
this, 4 xy's and I were to take away 4 xy's,
how many xy's am I left with? Well, I'm left with no xy's. Or you could say add
the coefficients, 4 plus negative 4,
gives you 0 xy's. Either way, these
two cancel out. If I have 4 of something
and I take away those 4 of that something,
I'm left with none of them. And so I'm left with no xy's. And then I have
right over here-- I could have written
0xy, but that seems unnecessary--
then right over here I have my x squared terms. Negative 2 plus 3 is 1. Or another way of saying
it, if I have 3x squared and I were to take
away 2 of those x squared, so I'm left
with the 1x squared. So this right over here
simplifies to 1x squared. Or I could literally
just write x squared. 1x squared is the same
thing as x squared. So plus x squared,
and then these there's nothing really left to simplify. So plus 2x plus y squared. And we're done. And obviously you
might have gotten an answer in some other
order, but the order in which I write these
terms don't matter. It just matters that you
were able to simplify it to these four terms.