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## 6th grade

### Unit 6: Lesson 13

Combining like terms# Combining like terms example

We're going to simplify this expression together putting to use our new knowledge of how to combine like terms. Ok? Let's do it! Created by Sal Khan.

## Want to join the conversation?

- why is X the most common letter used in math?(18 votes)
- Here is one perspective on this Ted Talk - "Why is X the unknown,

http://www.ted.com/talks/terry_moore_why_is_x_the_unknown?language=en(8 votes)

- Can we actually combine terms like that? First, it was in the right order and then Sal changed the order to gather same species. But I don't know if ...(7 votes)
- Yes you can!

The mathematical property which allows us to do so is the commutative property of addition, which says, essentially, that, "when adding things up, order doesn't matter," so x+y+z=x+z+y=z+y+x etc.

So if I have 2x + 3y + 4z - x - 2y - 3z, I can rearrange that to 2x - x + 3y - 2y + 4z - 3z.

Then I can combine the like terms, shown with parenthesis: (2x - x) + (3y - 2y) + (4z - 3z)

2x - x = x

3y - 2y = y

4z - 3z = z

Now where there is the 2x - x I can replace that with x: x + (3y - 2y) + (4z - 3z)

Where there is the 3y - 2y I can replace that with y: x + y + (4z - 3z)

and finally the z terms to get x + y + z which is exactly equal to the original expression, that is:

2x + 3y + 4z - x - 2y - 3z = x + y + z.(8 votes)

- i don t get what minus one z from 8 z and it equals 7 how? this happens around2:50to3:00(6 votes)
- It's 8z - z. Think of it as 8 - 1. z is also just another way of saying 1z.(5 votes)

- so i'm confused with this question:

Combine the like terms to create an equivalent expression.

4t−t+2

and it said the answer is this:

4t-t+2=(4-1)t+2

= 3t+2

I am confused where did the (4-1) come from? I understand where the 4 is from but where did the 1 come from? I dont see any 1 inside the question(4 votes)- Remember, a variable without a visible number in front has a coefficient of 1. So, your problem is actually:

4t-1t+2

Combine the 2 terms containing "t" by subtracting their coefficients and you get 3t+2

Hope this helps.(4 votes)

- Why do i hate khan(4 votes)
- Only you can answer that, what is your attitude toward Math in general? Do you feel like you are improving or just getting by? Khan has a lot of good content that help a lot of other people, so you have to figure why it does not help you. Or maybe you are upset because someone (a teacher or parent) is making you do this, and you just protest against others telling you what to do. I cannot answer your question.(3 votes)

- how would , for example 2z-7-1 = 2z + 8(4 votes)
- if your are able to do it right you are able to have the variable a zero(4 votes)

- How do you Combine the like terms to create an equivalent expression? Like for example 4p +6 −3 how would you solve that?(3 votes)
- 4p+3 since you can combine the +6 and the -3 into +3. You can't combine any further as p can be anything and there are four of them.(5 votes)

- Okay now I've watched this and I'm still a little confused(4 votes)
- well you are just add the X's to the numbers like this (the first number is the coefficient btw)2x +4X = whatever the answer would be.(2 votes)

- when I watching this video this looks so easy but when I taking the test it's really hard!

like this one - 4q - ( - 8q) + 10

I thought the answer is - 12q + 10

bcoz the the rule is "negative minus negative" the number will be just get more negative?(2 votes)- No, the number wont get more negative.

When there is an equation like this -4q - (-8q) +10, where there is a minus sign in front of the brackets, the sign(s) inside the brackets get changed. So the equation becomes this: -4q +8q +10.

So the answer is 4q+10(3 votes)

- I understand that like terms have to go together. (ie. x goes with x, y goes with y, etc.) My question is, how do I know where the addition and subtraction symbols go? When they're all spread throughout the problem?(3 votes)
- Maybe it will help you think about them as coefficients of the variables, so if you have 3x-2x, you add the coefficients of 3 + -2. If it is more complicated such as 4x - 3y + 2z - 4y + 3x - 4z + x -2y - 3z x has coefficients of 4, 3, and -3, y has coefficients of -3, -4, and -3, and z has coefficients of 2, -4, and -3. In each case, add the 3 numbers together.(1 vote)

## Video transcript

We have a hairy-looking
expression here. And your goal is to try to
simplify it as much as you can. And I'll give you a little
bit of time to do it. Let's just think about
it, step by step. And it might help if we
were to actually reorder the terms in this expression. So let me put all
the x terms first. So I have 5x-- that's
that term-- minus 2x. Then I have plus 7y plus 3y. Then i have plus 8z,
and then I have minus z. And then the last term
that I haven't included yet is that plus 5. Now we'll just think it through. If I have 5 x's and
I were to take away 2 x's, is how many x's am
I going to be left with? Well, I'm going to
be left with 3 x's. That's true of anything. There's not some fancy
algebraic magic going on here. 5 of anything minus
2 of that same thing, you're going to be left
with 3 of that thing. In this case, that
thing are x's. So this is going
to simplify to 3x. Now, in a lot of
algebra classes, you'll hear people say,
oh, well, you know, the coefficient on 5x is 5. And the coefficient on
this subtracting the 2x, the coefficient
here is negative 2, and we had to add
the coefficients. Let me write that word
down-- coefficient. These right over here
are the coefficients. They're the number that you're
multiplying the variable by. So you're the 5 or the
negative 2 in this case. And so you could
just say, oh, I had to just add the coefficients. And that's OK, and there's
nothing wrong with that. But I really want to
emphasize that there's a very common sense
intuition here. If you have 5 of something, you
take away 2 of that something, you are left with 3
of that something. And you have to be very careful. You have to make sure that
you're adding and subtracting the same things. Here, we're dealing with x's. So we can take 5 x's
and take away 2 x's. We can't think about merging
the x's and the y's, at least not in any simple way right
now, because that, frankly, wouldn't make any
intuitive sense. Now let's think about the y's. If I have 7 of something,
and I were to add 3 more of that something,
well, then, I'm going to have 10
of that something. So this part right over here
is going to simplify to 10y. Once again, you could say
the coefficient on 7y is 7. The coefficient on 3y is 3. We added the coefficients--
7 plus 3-- to get 10y. But I really want to
emphasize the intuition here. It's much more if you've got 7
of something, you add another 3 to that something, you've
got 10 of that something. Now let's look at the z's. If I've got 8 of something
and I take away 1 of them, I'm going to have 7
of that something. So that is 7z. And you might say, hey, wait. What was the coefficient
right here on this negative z? I don't see any number
out front of the z. Well, implicitly, I
could have put a 1 here, and it's exactly the same thing. Subtracting a z is the exact
same thing as subtracting 1z. The word "onesie" strikes
a part of my brain because I have very
young children, but that's a different
type of onesie. And then you could
see, oh, yeah, you definitely did add the
two coefficients, the 8 and the negative 1. But once again,
common sense tells you if you have 8 of something,
and you take away 1 of them, you have 7 of that something. And then finally,
you have a plus 5. So we're done. This simplified to 3x
plus 10y plus 7z plus 5.