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## Algebra 1

# Intro to factors & divisibility

CCSS.Math: ,

Sal explains what it means for a polynomial to be

*of another polynomial, and what it means for a polynomial to be***a factor***another polynomial.***divisible by**## Video transcript

- [Voiceover] You're probably
familiar with the general term factor. So if I were to say: What
are the factors of 12, you could say: Well what
are the whole numbers that I can multiply by another
whole number to get 12? So for examples, you
could say things like, well I could multiply
one times 12 to get 12. So you could say that
one is a factor of 12. You could even say that
12 is a factor of 12. You could say two times
six is equal to 12, so you could say that
two is a factor of 12 and that six is also a factor of 12. And of course three times four is also 12, so both three and four are factors of 12. So if you said well what
are the factors of 12 and you've seen this
before, well you could say: one, two, three, four, six, and 12, those are all factors of 12. And you could also phase
it the other way around, so let me just give an example. So if I were to pick on
three, I could say that three is a factor of 12. Or to phrase it slightly
differently, I could say that 12 is divisible, 12 is divisible by three. Now what I wanna do in this
video is extend this idea of being a factor or divisibility
into the algebraic world. So for example, if I were to take 3xy. So this is a monomial with
an integer coefficient. Three is an integer right over here. And if I were to multiply
it with another monomial with an integer coefficient, I don't know, let's say times negative two X squared, Y to the third power, what is this going to be equal to? Well this would be equal to, if we multiply the coefficients,
three times negative two is going to be negative six. X times X squared is X to the third power. And then Y times Y to the third is Y to the fourth power. And so what we could say is,
if we wanted to say factors of negative six X to the
third, Y to the fourth, we could say that 3xy is a factor of this just as an example; so
let me write that down. We could write that 3xy is a factor of, is a factor of... of negative six X to the
third power, Y to the fourth, or we could phrase that
the other way around. We could say that negative
six X to the third, Y to the fourth, is divisible by, is divisible by 3xy. So hopefully you're seeing the parallels. If I'm taking these two monomials
with integer coefficients and I multiply 'em and I get
this other, in this case, this other monomial, I could
say that either one of these and there's actually
other factors of this, but I could say either
one of these is a factor of this monomial, or we
could say that negative six X to the third, Y to the four is divisible by one of its factors. And we could even extend this
to binomials or polynomials. For example, if I were to take, if I were to take, let me scroll down a little bit, whoops, if I were to take, let me say X plus three and I wanted to multiply it times X plus seven, we know that this is going to be equal to, if I were to write it as a trinomial, it's gonna be X times X, so X squared, and then it's gonna be
three X plus seven X, so plus 10x; and if any
of this looks familiar, we have a lot of videos
where we go in detail of multiplying binomials like this. And then three times seven is 21. Plus 21. So because I multiplied these
two, in this case binomials, or we could consider
themselves to be polynomials, polynomials or binomials
with integer coefficients. Notice the coefficients
here, they're one, one. The constants here, they're all integers. Because I'm dealing with all
integers here, we could say that either one of these
binomials is a factor of this trinomial, or we could say this
trinomial is divisible by either one of these. So let me write that down. So I could say, I'll just
pick on X plus seven. We could say that X plus seven is a factor, is a factor of X squared plus 10x plus 21; or we could say that X squared plus 10x plus 21 is divisible by, is divisible by I could say X plus three
or I could say X plus seven is divisible by, X plus seven. And the key is, is that
both of these binomials, or even if we were
dealing with polynomials, we are dealing with
things that have integer, we're dealing with things that
have integer coefficients.