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Solving quadratics by factoring

Learn how to solve quadratic equations like (x-1)(x+3)=0 and how to use factorization to solve other forms of equations.

What you should be familiar with before taking this lesson

What you will learn in this lesson

So far you have solved linear equations, which include constant terms—plain numbers—and terms with the variable raised to the first power, x1=x.
You may have also solved some quadratic equations, which include the variable raised to the second power, by taking the square root from both sides.
In this lesson, you will learn a new way to solve quadratic equations. Specifically you will learn
  • how to solve factored equations like (x1)(x+3)=0 and
  • how to use factorization methods in order to bring other equations (like x23x10=0) to a factored form and solve them.

Solving factored quadratic equations

Suppose we are asked to solve the quadratic equation (x1)(x+3)=0.
This is a product of two expressions that is equal to zero. Note that any x value that makes either (x1) or (x+3) zero, will make their product zero.
(x1)(x+3)=0x1=0x+3=0x=1x=3
Substituting either x=1 or x=3 into the equation will result in the true statement 0=0, so they are both solutions to the equation.
Now solve a few similar equations on your own.
Solve (x+5)(x+7)=0.
Choose 1 answer:

Solve (2x1)(4x3)=0.
Choose 1 answer:

Reflection question

Can the same solution method be applied to the equation (x1)(x+3)=6?
Choose 1 answer:

A note about the zero-product property

How do we know there are no more solutions other than the two we find using our method?
The answer is provided by a simple but very useful property, called the zero-product property:
If the product of two quantities is equal to zero, then at least one of the quantities must be equal to zero.
Substituting any x value except for our solutions results in a product of two non-zero numbers, which means the product is certainly not zero. Therefore, we know that our solutions are the only ones possible.

Solving by factoring

Suppose we want to solve the equation x23x10=0, then all we have to do is factor x23x10 and solve like before!
x23x10 can be factored as (x+2)(x5).
The complete solution of the equation would go as follows:
x23x10=0(x+2)(x5)=0Factor.
x+2=0x5=0x=2x=5
Now it's your turn to solve a few equations on your own. Keep in mind that different equations call for different factorization methods.

Solve x2+5x=0.

Step 1. Factor x2+5x as the product of two linear expressions.

Step 2. Solve the equation.
Choose 1 answer:

Solve x211x+28=0.

Step 1. Factor x211x+28 as the product of two linear expressions.

Step 2. Solve the equation.
Choose 1 answer:

Solve 4x2+4x+1=0.

Step 1. Factor 4x2+4x+1 as the product of two linear expressions.

Step 2. Solve the equation.
Choose 1 answer:

Solve 3x2+11x4=0.

Step 1. Factor 3x2+11x4 as the product of two linear expressions.

Step 2. Solve the equation.
Choose 1 answer:

Arranging the equation before factoring

One of the sides must be zero.

This is how the solution of the equation x2+2x=40x goes:
x2+2x=40xx2+2x40+x=0Subtract 40 and add x.x2+3x40=0Combine like terms.(x+8)(x5)=0Factor.
x+8=0x5=0x=8x=5
Before we factored, we manipulated the equation so all the terms were on the same side and the other side was zero. Only then were we able to factor and use our solution method.

Removing common factors

This is how the solution of the equation 2x212x+18=0 goes:
2x212x+18=0x26x+9=0Divide by 2.(x3)2=0Factor.x3=0x=3
All terms originally had a common factor of 2, so we divided all sides by 2—the zero side remained zero—which made the factorization easier.
Now solve a few similar equations on your own.
Find the solutions of the equation.
2x23x20=x2+34
Choose all answers that apply:

Find the solutions of the equation.
3x2+33x+30=0
Choose all answers that apply:

Find the solutions of the equation.
3x29x20=x2+5x+16
Choose all answers that apply:

Want to join the conversation?

  • blobby green style avatar for user Amber Lynn
    In the above equation 3x^2+11x-4 = 0, I understand where we need to find two numbers were a+b need to equal 11 to satisfy the 11x, however, I'm having trouble connecting where -12 came from where it states that we need to find numbers to satisfy (a)(b) = -12. I'm seeing a -4 at the end of the equation. Not sure where -12 came from. Was it from multiplying -4 to the co-effiecient of the 3 in 3x^2?
    (46 votes)
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    • aqualine ultimate style avatar for user Johnathan
      In the standard form of quadratic equations, there are three parts to it: ax^2 + bx + c where a is the coefficient of the quadratic term, b is the coefficient of the linear term, and c is the constant. The -4 at the end of the equation is the constant. This hopefully answers your last question. Now, your first question.

      So the problem is that you need to find two numbers (a and b) such that the sum of a and b equals 11 and the product equals -12. Correct? Correct indeed. I use a pretty straightforward mental method but I'll introduce my teacher's method of factors first. What you need to do is find all the factors of -12 that are integers. Let's start with 1.
      1 * -12
      2 * -6
      3 * -4
      4 * -3
      6 * -2
      12 * -1
      Here we see 6 factor pairs or 12 factors of -12. Let's see which one adds up to 11. It seems like 12 + (-1) = 11. So we'll split 11 to 12 and -1.
      My other method is straight out recognising the middle terms. This works well with small numbers. I can clearly see that 12 is close to 11 and all I need is a change of 1. So that leaves out 12 * -1 and -12 * 1. I can see that -12 * 1 makes -11 which is not what I want so I go with 12 * -1.

      If you misunderstand something I said, just post a comment.
      (36 votes)
  • male robot johnny style avatar for user Jose Santizo
    Sometimes I don't understand some of the problems. :(
    (26 votes)
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  • hopper cool style avatar for user Clark Burch
    still confused
    (13 votes)
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    • stelly blue style avatar for user Kim Seidel
      Think about what is confusing you.
      1) Is it the factoring? If it is, you need to review the lessons and practice problems for factoring. There are multiple factoring techniques. You need to know each of them and when to apply them.

      2) If you are ok with the factoring, then what pay close attention to the other topics on the page. They cover the other steps you need to do in quite some detail.
      (14 votes)
  • marcimus pink style avatar for user Cooky_0901
    I don't get 3x^2-9x-20=x^2+5x+16. What do we do in the first place?
    (4 votes)
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  • blobby green style avatar for user gmblim
    In the above equation 3x^2+11x-4 = 0, I understand where we need to find two numbers were a+b need to equal 11 to satisfy the 11x, however, I'm having trouble connecting where -12 came from where it states that we need to find numbers to satisfy (a)(b) = -12. I'm seeing a -4 at the end of the equation. Not sure where -12 came from. Was it from multiplying -4 to the co-effiecient of the 3 in 3x^2?
    (5 votes)
    Default Khan Academy avatar avatar for user
  • piceratops seedling style avatar for user Danny
    I know this is not related to the above questions, but i could not find where to ask normal day to day questions. If someone could please help me answer this would be great.:
    Factorise the expression as far as possible to prime factors:
    (x-1)^2-9(y-1)^2 ...
    ^2(power of 2)
    (2 votes)
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  • duskpin sapling style avatar for user Anushka
    How do you factor it when the leading coefficient is more than 1? For example, something like 3xsquared+ x-2.
    (4 votes)
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  • aqualine sapling style avatar for user Laney
    My equation is x^2-2x+1=0 ( find x) How do i factor out the 1? Could i make it 1^2?
    (4 votes)
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  • spunky sam blue style avatar for user 💎Chυcκ Lørrε💎
    I figure out that when A+B=[some number] and AB=[some number] combines, it could be an linear equation. But I don't know how to solve it, because when I'm solving it, a new quadratic equation comes out! For example:
    Problem:
    Factoring x²+3·x-10=(x+a)(x+b)
    Linear equation:
    ①a+b=3
    ②a·b=-10
    Solve:
    a=3-b
    Substitute 3-b into equation ②
    (3-b)b=-10
    3·b-b²=-10
    And I don't know how to solve.
    (3 votes)
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    • leaf red style avatar for user p.m.
      That's a confusing way to factor, because as you noticed it just results in another quadratic equation. It's easier to just try to think of 2 numbers that add to 3 and multiply to -10, than actually solve it. You could also use the quadratic equation, which always works, so you don't have to try to guess the factors. However, this often takes longer, and if the numbers are simpler you should factor instead.
      (1 vote)
  • duskpin ultimate style avatar for user R Dunkin
    Why do the numbers attached to the x disappear? for example: x^2 -20x + 100 = 0
    (x-10)^2 Where did the -20x go?
    (1 vote)
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    • stelly blue style avatar for user Kim Seidel
      Think about factors of numbers. 132 has factors of 11*12. Yet, the original number contains a 3. Where did it go? It only shows up if you multiply the factors. The same is true for factors of quadratics or other polynomials.
      Multiply (x-10)^2 and you will recreate the -20x.
      (x-10)^2 = (x-10)(x-10) x^2-10x-10x+100 = x^2-20x+100

      Hope this helps.
      (4 votes)