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## Algebra (all content)

### Course: Algebra (all content)ย >ย Unit 10

Lesson 18: Factoring quadratics: Perfect squares- Perfect square factorization intro
- Factoring quadratics: Perfect squares
- Perfect squares intro
- Factoring perfect squares
- Identifying perfect square form
- Factoring higher-degree polynomials: Common factor
- Factoring perfect squares: negative common factor
- Factoring perfect squares: missing values
- Factoring perfect squares: shared factors
- Perfect squares

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# Factoring perfect squares: missing values

CCSS.Math: , ,

Sal analyzes the factorization of x^2+5x+c as (x+d)^2 to find the values of the missing coefficients c and d.

## Want to join the conversation?

- At2:00, can someone explain how d is turned into 5/2? Also explain how 2d equals 5, and not 2dx.(14 votes)
- Hopefully, you can see that the 2 middle terms must equal. You can use: "2dx = 5x" or you can just use Sal's version: "2d = 5". If you solve either of these for "d", you will get "d = 5/2".

-- if you start with: "2dx = 5x", you need to divide by "2x" to solve for "d"

2dx / (2x) = 5x / (2x)

d = 5x / (2x) Reduce

d = 5/2

-- if you start with "2d = 5", just divide both sides by 2 and you get d = 5/2

Hope this helps.(22 votes)

- So perfect square pattern is just a shortcut method like cross multiply?

And the general form if using grouping method?(4 votes)- Yes... if you have a perfect square trinomial, you can use the pattern as a quicker way to do the factoring. The pattern can also be used to square 2 binomials because it creates the perfect square trinomial.(5 votes)

- This problem is so confusing. I can't comprehend how 5/2 would give us the answer. Arenโt the factors of d^2 suppose to equal 5x when added together?(4 votes)
- Can the answer be represented as a decimal?(2 votes)
- You could, but sometimes, it's just easier to answer in fractional form. :)(1 vote)

- Is this method used to solve linear equations?(1 vote)
- No, linear equations are 1st degree (highest exponent = 1). Factoring is used for 2nd degree and higher equations.(4 votes)

- At0:13sal says that the numbers are posotive rational numbers. What does this mean?(2 votes)
- Positive means it is not a negative number and rational means is a number that does not have a decimal that goes on forever. Sal is basically saying this question is playing by the rules as we know at this point and there is no weird stuff going on.(1 vote)

- I'm kind of confused like what will I do for a question like 9x^2 + 6x +1?(1 vote)
- Is 9x^2 a perfect square? Yes. It is (3x)^2

Is 1 a perfect square? Yes. It is 1^2

Does 2(3x)(1) = 6x? Yes.

So, this is a perfect square trinomial.

The factors are: (3x+1)(3x+1) or (3x+1)^2

Hope this helps.(2 votes)

- Sal explains we can "pattern match" to figure out why 2d = 5 https://youtu.be/oIslHyWJHEY?t=91(0 votes)

- c doesn't equal d. c=d^2, because x^2+5x+c=x^2+2dx+d^2 and the terms without x must equal each other.(1 vote)

- What if I just have an equation X^2+5x+c(1 vote)

## Video transcript

- [Voiceover] The quadratic expression x-squared plus five x plus c is a perfect square. It can be factored as x plus d-squared. Both c and d are positive
rational numbers. What I wanna figure out in this video is what is c, given the information that we have right over here? What is c going to be equal to? And what is d going to be equal to? Like always, pause the video and see if you can figure it out. Let's work through this together. We're saying that x-squared
plus five x plus c can be rewritten as x plus d-squared. Let me write that down. So this part, this part, x-squared plus five x plus c, we're saying that, that could be written as x plus d-squared. This is equal to x plus d-squared. Now we can rewrite, x plus d-squared is going to be equal to x-squared plus two dx plus d-squared. If this step, right over
here, you find strange, I encourage you to watch the videos on squaring binomials or on
perfect square polynomials, either one, so you can see the pattern that this is going to be. X squared plus two times the product of both of these terms plus d-squared. When you look at it like this, you can start to pattern
match a little bit. You can say, alright,
five x, right over here, that is going to have
to be equal to two d, and then, you can also say, that c is going to have
to be equal to d-squared. Once again, you can say
two d is equal to five, two d is equal to five, or that d is equal to five halves. We've figured out what d is equal to. Now we can figure out what c is, because we know that c needs
to be equal to d-squared, gimme that orange color, actually, so we know that c is equal to d-squared, which is the same thing
as five halves, squared. We just figured out what d is equal to. Gonna be five halves, squared, which is going to be 25 over four. C is equal to 25 over four, d is equal to five halves. We're done.