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Describing numerical relationships with polynomial identities

Polynomial identities can be used to explain or describe various things in math. For example, we can use identities to prove that the differences between consecutive square numbers (1,4,9,...) are consecutive odd numbers (1,3,5,...).

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  • blobby green style avatar for user Mim Tahze
    I think I need some help to understand what is going on. As I understood the difference between any number by it's previous number is 2n+1. Then, let's take 9.
    2(9) + 1 = 19
    but the difference between 9 and 4 is not 19. do you guys have any idea?
    (5 votes)
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    • leaf orange style avatar for user A/V
      The polynomial '2n+1' is defined as the difference between any number by it's previous number. You have the right idea; however, we're looking at what is inbetween. To clarify:

      The sequence is n², and you want to put 9.
      9² = 81
      Then,
      10²=100

      The goal of '2n+1' is the difference between (in this example), 10²-9² = 100-81 = 19, your answer.

      Another example is that you want to plug in 13 and 14 for n², and you want to find the difference:
      13² = 169
      14² = 196,

      Using our formula,
      2(13)+1 = a 27 number distance. Let's now check:

      14²-13²=27
      (14 votes)
  • hopper cool style avatar for user jeg906
    How would you use this in real life? Economics maybe?
    (3 votes)
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    • primosaur ultimate style avatar for user Matthew
      I'm no expert, but I have heard that sequences (such as the ones shown in this video) show up in computer programming. They also show up in calculus and statistics, which is used in real life for sure. Also, algebra as a subject overall improves your critical thinking skills, which is important to have in life.
      (18 votes)
  • blobby green style avatar for user pjdequena
    On the second example, how do you know that n is n^2?
    (4 votes)
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    • primosaur ultimate style avatar for user Matthew
      Because the second sequence is describing the numbers in the first sequence squared. n is the value of any number in the first sequence, so n^2 is the value of any number in the first sequence squared (which is what the second sequence describes).
      (2 votes)
  • blobby green style avatar for user zjleon2010
    I think in real life, someone will just throw the second sequence right on your face and said find some pattern for me!
    to be honest, if I encounter such circumstance, I may not find the n^2 pattern
    (3 votes)
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  • piceratops ultimate style avatar for user Austin Sevier
    at I noticed it was a +1 following the n^2 + 2n but shouldn't it be +2?
    (1 vote)
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  • blobby green style avatar for user Anas Mashhood
    What does the 2n+1 represents?
    (1 vote)
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  • blobby green style avatar for user Anas Mashhood
    *May somebody please explain to me what Sal just did? I am so confused. Thank you.*
    (1 vote)
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    • aqualine seedling style avatar for user Alexia
      Sal showed the pattern of differences between consecutive square numbers. And then, he uses the polynomial identities to explain how we can find out the difference of two random consecutive positive integers.
      Hope you understood
      (1 vote)
  • male robot hal style avatar for user Vinay Sharma
    what else can we prove with this knowledge in our yard?
    (1 vote)
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  • duskpin ultimate style avatar for user Kiba
    I have two questions:

    1. How does an identity help us with proving this? I didn't quite get an understanding of that in the video. Why an identity exactly?

    2. How did Sal know to set up the equation as (n+1)^2-n^2=n^2+2n+1?
    I see that it's an unfactored form of the left side of the equation, but why is it necessary? How would we know that the n^2+2n+1 part is needed? What does this form explain better that makes it preferred in this form instead of just having the left side copied on the right side as well?


    I'm sure you can tell that I didn't quite understand the video much.
    (1 vote)
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  • mr pants purple style avatar for user Zidong Qiu
    how to use hyperbolic functions plz help
    (1 vote)
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Video transcript

- [Instructor] What we're going to do in this video is use what we know about polynomials and how to manipulate them and what we've talked about of whether two polynomials are equal to each other for all values of the variable that they're written in, so whether we're dealing with a polynomial identity. And we're going to use those skills in order to prove some properties of relationships between numbers. So if I were to list out some integers, I could go zero, I could go one, I could go two, three, four, five. And if I were to list out the squares of these, if I were to create a sequence of integer squares, well, zero squared would be zero, one squared would be one, two squared is four, three squared is nine, four squared is 16, five squared is 25. And we could, of course, keep going in either case. But the first thing I want you to think about, before you even write down a polynomial or try to construct one, is look at this sequence of integer squares. And do you see any pattern in terms of the difference between successive terms of this sequence of integer squares? All right, now let's think about this together. So to go from zero to one, you add one. And to go from one to four, you add three. To go from four to nine, you add five. To go from nine to 16, you add seven. It seems like a pattern here. As we go to successive terms of this sequence of integer squares, we're adding increasing odd numbers. So I'm guessing that if I add nine here, which is the next odd number, I'm gonna get to 25, and that indeed is the case. And you could test that out. Well, what, if I add 11, which would be the next odd number, what do I get to? I get to 36, which is the square of six. But how can we feel good that this always is true, that this never breaks down? Well, one way to do it is to think a little bit more generally, and that's where our algebra and our knowledge of polynomials are going to be useful. So let's say we go all the way, and we're just speaking generally now. So we have the number n, and then with the next number after that is going to be n plus one. And then if we think about what the corresponding terms in the sequence of integer squares would be, well, that would be, when we square it, when we get to n, we would get n squared. And when we get to n plus one, we would have n plus one squared. And let's see if we could think about what the difference between these two things are. The difference between 25 and 16 is nine. Difference between 16 and nine is seven. So let's think about what the difference between n plus one squared and n squared is. And how do we write that as a polynomial? Well, it'll just be n plus one squared minus n squared. And now let's see if we can rewrite this, algebraically manipulate this so we can set up a polynomial identity that describes this pattern that we just saw. So what I'll do is I'm just going to expand out n plus one squared right over there. So that is going to be n squared plus two n plus one. And then we have this minus n squared here, so minus n squared. And so we see that n squared minus n squared cancels out. And so we can rewrite everything we have here as n plus one squared minus n squared. So this is really the difference between successive terms in our sequence of integer squares is going to be equal to two n plus one for any integer n. Well, for any integer n, what is two n plus one going to be? And especially here, we're dealing with the positive integers. Well, for any integer n, this is going to be an odd integer. If you take any integer, you multiply it by two, this part is going to be even. But then you add one to that, you're going to get an odd integer. And you can also see that this increases by two as n increases. So when you go from one odd integer, you go add two to the next odd integer. You add two to the next odd integer, which is exactly what is described there. So this is pretty neat. We've just used a little bit of algebra, a little bit of what we know about polynomial identities to show that the difference between successive terms in this sequence of integer squares right over here is going to be increasing odd numbers.