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## Finding zeros of polynomials

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# Finding zeros of polynomials (example 2)

CCSS.Math: , , ,

## Video transcript

- So I have the polynomial p of x here, and p of x is being expressed
as a fourth degree polynomial times three x minus eight squared, so this would actually give you some, this would give you nine x squared and a bunch of other stuff, and then you multiply that times this. It would actually give you a sixth degree polynomial all in all, but our goal is to find the x values where that makes p of x equal to zero, or another way find the roots or the zeros of this polynomial, and in particular we're going
to focus on the real zeros, the real roots of this polynomial, and like always I encourage
you give a go at it, and then we'll do it together. Alright, let's tackle this. So, the way I want to solve
p of x is equal to zero. I want to solve p of x is equal to zero, and figure out what, and when I say solve it I want to say well what x values will make
the polynomial equal to zero. So I just need to set this right hand side to be equal to zero and then solve for x, and the best way that I
can think of doing that is by factoring this out as much as I can, and if I can rewrite it as a product of a bunch of
expressions equaling zero, well a product of a bunch
of things equaling zero, you can make it equal zero by
any one of them equaling zero, and so let's do that. So three x minus eight squared, this is already factored quite nicely, let's see if we can
factor if we can factor all of this business in white, and the way I will tackle it to see if I can factor by grouping. So let me group together
these first two terms, and then let me group together
these second two terms, and essentially factoring by grouping is doing the distributive
property in reverse twice. So from these first two terms, I could factor out, let's see what could I factor out? I could factor out a let me see, I'll just factor out an
x to the third power. So I get x to the third power times three x minus eight, interesting we have a three x
minus eight over there as well Now these second to two terms, I could factor out a five, so this is going to be plus five times x, sorry, times three x. Three x minus eight, very interesting, and of course I have these
parentheses around all of that, and then I have three x minus 8 squared. This three x minus eight
is showing up a lot, and so and of course this is
going to be equal to zero. So we're gonna be equal to zero, and now I can factor out a
three x minus eight over here, I could factor that
three x minus eight out, and I'm going to get three x minus eight times times x to the third power, x to the third power plus five. Once again I just factor
out a three x minus eight plus five, close the parentheses, and then times three x minus eight squared is all going to be equal to zero, is all equal zero. Now if what I just did
looks a little like voodoo, just realize I have two terms, both of them are multiples
of three x minus eight. I just factored out, I just factored out the
three x minus eight. I did distributive property in reverse, so I factored it out, and what you're left with this term you just look for the next of third, and in this term you're
just left with a plus five. Now three x minus eight times
x to the third plus five times three x minus eight squared. Well I could just rewrite this as three x minus eight to the third power times x to the third plus five, so let me do that. So I can just rewrite this as three x minus eight to the third power, that's that times that, and then times times, do this in a nicer color, times x to the third plus five is equal to zero, is equal to zero, and now in order to get
this to be equal zero, either the three x either this thing is going to be equal to zero, or this thing over here is
going to be equal to zero. So let's first think
about three minus eight to the third power equaling zero. So I can write this is as three x minus eight to the
third power is equal to zero or x to the third plus
five is equal to zero. So to make three x minus eight
to the third power equal zero well that means three x minus eight is going to be equal to zero or that three x is equal to eight, divide both sides by three, x is equal to eight thirds. So that's one way to make
this polynomial equal zero, x is equal to eight thirds, in fact just this right
over there will become zero, zero times anything is zero. So this is a zero of our polynomial, and let's see, so x to the third, so we could say, if we subtract five from both sides, we have x to the third is equal to the is equal to negative five, and so if we take both
to the one third power we could say x is equal to
cube root of negative five. Now at first you might say, "Wait, can I take the square
root of a negative number?" and I would say, "Of course you can!" The cube root of a negative
one is negative one. The cube root of negative
eight is negative two. In fact you could, even if
we're dealing with reals. This is going to be a negative number. This is not going to be an
imaginary number right over here, and so these are, these are the two zeros of the polynomial. There's gonna be negative, I think, negative one point something, I'm sure we could figure out it, figure out it exactly. So let's raise, so let's raise, five to the open parentheses, one divided by three close parentheses, is equal to, so that's five to the one third, so negative five to the one third power is gonna be negative one
point seven one approximately. So this is approximately equal
negative one point seven one. So we have two real roots, two real roots to this polynomial, or two zeros two real
zeros for this polynomial, and so those are going
to be the two places where we intercept the x-axis. The two x values for which where the two places where
we intercept the x-axis, is the easiest way to say it.