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## Algebra (all content)

### Course: Algebra (all content)ย >ย Unit 5

Lesson 5: Manipulating expressions with unknown variables# Manipulating expressions using structure (example 2)

CCSS.Math:

Given that a+b=2a, Sal finds an expression equivalent to b-a.

## Want to join the conversation?

- Why do we manipulate expressions in math and what is it used for in reality?(19 votes)
- We manipulate expressions for several reasons. Here are three of them.

One reason is to make the expression easier to work with. Sometimes an expression might be written in a more complicated way than is necessary. We can manipulate the expression to change it into a simpler form.

A second reason is to highlight an important aspect of the expression. For example, I might have an equation for a line written as "3x - 4y = 5". If I write the equation this way, I can quickly find the x-intercept and the y-intercept. But it's not so easy to see what the slope is. I can manipulate that equation to rewrite t as "y = 3/4(x) - 5/4". If I do, now I can easily see that the slope is 3/4.

A third reason is to get the expression to fit a formula. For example, the formula for difference of squares is "a^2 minus b^2 equals (a+b) times (a-b)". That's a great tool for getting rid of some pesky exponents. But the tool only works if you have two quantities that are both squares of something.

Could I use the difference of squares on an expression like "x^2 + 12x + 32"? At first, it doesn't look like a difference of squares. But maybe I can manipulate the expression to make it fit the formula for difference of squares. Let's try.

x^2 + 12x + 32

x^2 + 12x + 36 - 4 [replacing "32" with "36 - 4"]

(x^2 + 12x + 36) - 4 [associative property]

(x+6)^2 - 4 [factoring the perfect square trinomial]

(x+6)^2 - 2^2 [replacing "4" with "2^2"]

Now it's in the right format for difference of squares. "(x+6)" is the "a", and "2" is the "b".

(x+6 + 2)(x+6 - 2) [difference of squares]

(x+8)(x+4) [subtraction]

By manipulating the expression, I was able to turn "x^2 + 12x + 32" into "(x+8)(x+4)". That's good, because now I can easily find the two places where a graph of the expression will cross the x-axis.(19 votes)

- Couldn't this also be solved in this way:

a+b=2a

a+b-a=2a-a (subtract a from both sides)

Answer b=a

Next, if b=a we can either subtract b from both sides or a from both sides

Example of subtracting a from both sides : b-a=a-a thus b-a =0, or

subtracting b from both sides of the equation :

b-b=a-b thus a-b=0

a-b is the answer becuase it is equal to b-a

No negatives involved, am I wrong about this? Please comment if I am so I can correct my mistake. Ty(9 votes)- Your method is fine!

Good thinking - Nicely done.

Now, I am not too sure what you mean by "no negatives involved", or why you think that is even a concern, but be that as it may, remember there are many different ways to arrive a a solution. Some solutions may appeal more to you than others; there is always something to be gained by looking at a problem from as many perspectives as possible.(3 votes)

- i'm still quite confused by this 'manipulating expressions using structure' module.Can somebody explain to me how to solve this question again?(4 votes)
- Why did Sal multiply both sides by -1 to get the answer?(2 votes)
- Remember that an equation is a balance. It means literally 'equivalent' (an association).

So when you say: a = 2 you can also say that 2 = a , because they are equivalent.

Now imagine that: If you put a rock in one plate of a balance, you need to put a rock on the other plate as well to keep the balance plates stabilized at the same height.

That said, if I have an equation like: "a = 2", and multiply 'a' by '-1', I need to make sure that '2' gets multiplied by '-1' as well in order to maintain the balance, so "-1*a = -1*2" is the same thing as "a = 2".

What Saul did was just the same (just to make the equation looks like some of the available choices, first he found that 'b' was equal to 'a', given that he found that "b-a = 0", but there were no choice like "b-a" or "0", so he multiplied both sides by -1 to get "-b+a=0" or "a-b=0".(2 votes)

- Am I confused or Sal? when its written b=a, its clear that even a-b=0 consequently b-a will be 0 too. Do we have to multiply it -1 in Grid In questions ?(2 votes)
- I never would've guessed to multiply by negative 1. Does this make me incompetent?(1 vote)
- No, not at all. . . .
. . . now you know a new method to try if you come across a similar situation.**AND**(3 votes)

- this uses the same format as linear equations but in variables?(2 votes)
- Wait. When he had

b-a=0 why didn't he just swap them because a=b and b=a?(1 vote)- You can't swap them because subtraction is not commutative. Of course since they are equal, swapping them won't change the answer, but it's still breaking the math rules, which makes your proof incomplete/incorrect.(3 votes)

- I have looked at all the tips and all the questions. So now I'm going to ask this question, how did you come up with multiplying by negative one and is that legal? If it is legal, why is it legal?(1 vote)
- When you have an equation, you can manipulate the parts by operations including multiplying by -1 as long as you do it on both sides, but with a 0, multiplying anything just gets zero

Lets do it another way mathematically

If we start from b - a = 0 , I can do opposites of subtracting b and adding a

b - a = 0

-b+a -b+a

0 = -b + a since addition is commutative, I can switch paces for a and b

0 = a - b

Since both are equal to zero, they must be equal to each other,

b - a = a - b

So multiplying by a negative 1 is equivalent to moving all the terms to the other side by subtraction or addition, and multiplying by -1 on both sides is much easier than to move everything

Hope this answers your question(2 votes)

- The entire video is about finding that equation but in the end it still =0 which does not make any mathematical sense. Can someone help make sense of it for me? Thanks.(1 vote)
- Well this particular problem asked which equation equaled b-a not what its value was. Think of it like saying what other equation are the same as lets say 10-15. Meaning what other equations wil have the same awnser.(2 votes)

## Video transcript

- [Voiceover] We're told, suppose a+b=2a. Which of these expressions equals b-a? Alright, I encourage
you to pause the video and see if you can figure that out. Which of these expressions would be equal to b-a and it's going to just involve
some algebraic manipulation. Alright, let's work through this together. So we are told, we are told that a+b=2a so the first thing I would want to do is get all my a's in one place and one way I could do that is I could subract a from both sides. So if I subtract a from both sides I'm going to be left with
just a b on the left-hand side and on the right-hand side
I'm going to be left with 2a-a, well that's just going to be a. If I have two of something
and I subtract one of them, take away one of them I'm going to have just one of those somethings equal to 1a. So we want to figure out what b-a is. Well luckily I can figure that out if I subtract a from both sides. So if I subtract a from both sides well then I'm going to
get on the left-hand side b-a, which is what we want to figure out, is equal to a-a=0. So b-a=0 which is not one of the choices. Alright, so let's see if we can figure out some other things over here. So b-a=0 but that is
not one of the choices. Alright, so let's see is there any other way to manipulate this? No, b minu- I could just go straight ahead and subtract 2a from both sides and I would get b-a=0. Oh, this is interesting,
this is a tricky one. So b-a=0. Well, if b-a=0 then if we take the negative of both sides of this, if we take the negative of both sides, if we multiply both sides by -1. So x, I should write the times,
I should write like this, alright, because we don't want to confuse it with the variable x, so if we multiply both sides by -1 what do we get? Well on the left-hand side we get a-b and on the right-hand side we still get 0. If b-a=0 then the negative of it, which is a-b is also
going to be equal to 0. And that's this choice. That is, let me do that
in a little darker color, that is this choice right over there. That was a good one.