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## Algebra 1

### Course: Algebra 1 > Unit 14

Lesson 8: More on completing the square- Solve by completing the square: Integer solutions
- Solve by completing the square: Non-integer solutions
- Solve equations by completing the square
- Worked example: completing the square (leading coefficient ≠ 1)
- Completing the square
- Solving quadratics by completing the square: no solution
- Proof of the quadratic formula
- Solving quadratics by completing the square
- Completing the square review
- Quadratic formula proof review

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# Solve by completing the square: Non-integer solutions

We can use the strategy of completing the square to solve quadratic equations even when the solutions aren't integers. Created by Sal Khan.

## Want to join the conversation?

- There doesn't seem to be any video on the practice: completing the square, and it just totally lost me with everything being added to the squared quadratic. Any tips would help, thanks!(7 votes)
- Try completing the square: intro and the videos associated with it.(2 votes)

- I have been trying at this for hours and The questions only throw more and more hurdles. nothing works for them.(3 votes)
- What exactly do you not understand?(2 votes)

- seriously I agree this is way too confusing to look at. I need some help(3 votes)
- With this trinomial, you need to review the previous lessons on completing the square if you don't understand this. Also, try again tomorrow! You only learn when it's hard and not too confusing!(2 votes)

- i plugged this into the quadratic formula. i got this answer:

-6±√36-36/2

-6±√0/2

-3±√0/2.

Where did I go wrong?(2 votes)- 4ac = 4(1)(3)=12, so it should be -6±√(36-12)/2. Then the √24=√4*√6=2√6.(2 votes)

- Why do we have to add +6 to both sides of the 0 = (x+3)^2 - 6 before taking the square root?

Can't we just take the square root without adding the +6 to both sides? Ex. Can't we do something like

√0 = √(x+3)^2 - 6

0 = x+3 +/- 6

+/-6 = x+3(1 vote)- In your example you’re not fully taking the square root of both sides. Note that all you did to the -6 was to change it to +/-. If you don’t add 6 to both sides you’d have to do √0 = √((x+3)^2 - 6) to keep both sides of the equation equal. It’s not very easy to take the square root of the whole expression (x+3)^2 - 6. Remember, you can’t distribute the square root. It’s easier just to add the six and then take the square root, getting +/-√ 6 = x+3, which turns to x = -3 +/-√6 once you subtract 3 from both sides.

I hope this helps!(3 votes)

- At2:10, I wish you did not leave out the detail to use the formula A^2+2AB+B^2=(A+B)^2 in the simplification process as it took me a lot of Googling to figure out that one step and it would not have been hard for you to state that rule in this video: "simplify using the rule A^2+2AB+B^2=(A+B)^2". You say it can be written as (x+3)^2, but not why. I need to know why in order to understand the whole problem. I was sitting here trying to figure out why I could not find a common denominator or how to multiply it to make it work. I wasted a lot of time because the video left out a simple detail. When you're applying a rule, please state the rule being applied.(1 vote)
- In a previous video, Sal did explain the formula and how to get the (A+B)^2. Here is the link:

https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:quadratic-functions-equations/x2f8bb11595b61c86:completing-square-quadratics/v/solving-quadratics-by-completing-the-square

I hope it helps!(1 vote)

- I am having trouble with this I NEED HELP!(1 vote)
- Many times you say "we add /some quantity/ inside the parenthesis and subtract some /multiplier/ * /some quantity/ outside the parenthesis. I don't understand what the multiplier is. Example: 2x^2 + 3x - 2. The square is 9/16 to complete the square (inside parenthesis), but subtract 2 times 9/16 outside the parenthesis. It looks like maybe the two is from the 2 in 2x^2. Is it to compensate for the 2 that was factored out? Another thing that is confusing is that sometimes things outside the parens are manipulated on the left side and at other times they are manipulated on the right side. I know that one can do this if the signs are correct, but it adds a little bit of confusion. On the other hand, I know I have to learn to deal with these things in the long run. Thanks for the lesson! By the way, the question was from a quiz after this lesson, I just had to find a place to ask a question before trying the quis again.(1 vote)
- Why is it +/- sqrt(6) instead of just square root of 6?(1 vote)
- Example: To find the square root of 9, we need to find a number that, when multiplied by itself, gives us 9. The reason why it is +/-3 is that both positive 3 and -3 give us 9.

-3 * -3 = 9

3 * 3 = 9

This works with all square roots (I think).(1 vote)

- In time stamp2:11, How come he did +9 and -9 for the Positive 3?(1 vote)
- He added the 9 in order to find the value of the square. He subtracted the 9 to keep the value of that side the same. Another way he could have done it is to add 9 to both sides, that way might have seemed more clear.(0 votes)

## Video transcript

- [Instructor] Let's say we're told that zero is equal to x
squared plus six x plus three, what is an x or what our x is that would satisfy this equation? Pause this video and try to figure it out. All right, now let's
work through it together. So the first thing that I would try to do is see if I could factor
this right hand expression, I have some expression,
it's equal to zero. So if I could factor it,
that might help solve. So let's see, can I think of two numbers that when I add them, I get six, and when I take their
product, I get positive three? Well, if I'm thinking
just in terms of integers, three is a prime number, it only has two factors one and three. And let's see one plus
three is not equal to six, so it doesn't look like factoring
is going to help me much. So the next thing I'll turn
to is completing the square. In fact, completing the square if there are x values that
would satisfy this equation, completing the square
will help us solve it. And the way I do it, I'll
say zero is equal to, let me rewrite the first
part, x squared plus six x, and then I'm gonna write
the plus three out here. And my goal is to add something to the right-hand
expression, right over here, and then I'm gonna
subtract that same thing, so I'm not really changing the
value of the right-hand side. And I wanna add something here that I'm later going to subtract, so that what I have in
parentheses is a perfect square. Well, the way to make it a perfect square, and we've talked about
this in other videos when we introduced ourselves
to completing the square, is we'll look at this first degree coefficient right over
here, this positive six, and say, okay, half of
that is positive three, and if we were to square
that, we would get nine. So let's add a nine there. And then we could also subtract a nine. Notice, we haven't changed the value of the right-hand side expression, we're adding nine and
we're subtracting nine. And actually, the parentheses
are just there to help it make a little bit more
visually clear to us, but you don't even need the parentheses, you would essentially get the same result. But then what happens if we
simplify this a little bit? Well, what I just showed you, let me do it in this green blue color, this thing can be rewritten
as x plus three squared. That's why we added nine there, we said, all right, we're
gonna be dealing with a three 'cause three is half of six. And if we squared three,
we get a nine there. And then this second
part, right over here, three minus nine, that's
equal to negative six. So we could write it like this, zero is equal to x plus
three squared minus six. And now what we can do is
isolate this x plus three squared by adding six to both
sides, so let's do that. Let's add six there, let's add six there. And what we get on the left-hand side, we get six is equal to,
on the right-hand side, we just get x plus three squared. And now we can take the
square root of both sides, and we could say that the plus
or minus square root of six is equal to x plus three. And if this doesn't make full sense, just pause the video a little
bit and think about it. If I'm saying that something
squared is equal to six, that means that the something is either going to be the
positive square root of six, or the negative square root of six. And so now we can, if
we wanna solve for x, we can just subtract
three from both sides. So let's subtract three from
both sides, and what do we get? We get on the right-hand side,
we just are left with an x, and that's going to be
equal to negative three plus or minus the square root of six. And we are done. And obviously we could
rewrite this and say, x could be equal to negative three plus the square root of six, or x could be equal to negative three minus the square root of six.