The polynomial 4x − 3 is a factor of the polynomial q x 4x^3 x 2 − 11x 2r what is the value of r

Video transcript

- So we have a polynomial here. What I'm curious about is what is the remainder if I were to divide this polynomial by, let's just say, x minus, I want the remainder when I divide this polynomial by x minus two? You could do this. You could figure this out with algebraic long division, but I'll give you a hint. It is much simpler and much less computation intensive and takes much less space on your paper if you use the polynomial remainder theorem. If that's unfamiliar to you, there's other videos that actually cover that. So why don't you have a go at it. All right, so now let's work through this together. The polynomial remainder theorem tells us that when I take a polynomial, p of x, and if I were to divide it by an x minus a, the remainder of that is just going to be equal to p of a. Is just going to be equal to p of a. So in this case, our p of x is this. What is our a? Well our a is going to be positive two. Remember it's x minus a. So let me do this. Our a is equal to positive two. So to figure out the remainder, we just have to evaluate p of two. So let's do that. So the remainder in this case is going to be equal to p of two, which is equal to, so let's see, it's going to be, I'll just do it all in magenta, negative three times eight minus, let's see, minus four times four plus 20 minus seven. So let's see, this is -24 minus 16 plus 20 minus seven. So that gives us, let's see, -24 minus 16, this is -40. All right, I'm just doing it step by step. This is equal to negative, actually I can do this in my head. All right, here we go. So this is -40 plus 20 is -20 minus seven is -27. That was pretty neat because if we attempted to do this without the polynomial remainder theorem, we would have had to do a bunch of algebraic long division. Now if we did the algebraic long division, we would have gotten the quotient and all of that, but we don't need the quotient, we don't need to know. So if we did all the algebraic long division, you know, we would have taken our p of x and then we would have divided the x minus a into it, and we would have gotten a quotient here, q of x, and we would have done all this business down here, all this algebraic long division. Probably wouldn't have even fit on the page. But eventually we would have gotten to a point where we got an expression that has a lower degree than this. It would have to be a constant because this is a 1st degree, so it would have be essentially a zero degree. So we would have eventually gotten to our -27. But this was much, much, much, much easier then having to go through this entire exercise. Hopefully you appreciated that.

Video transcript

- [Voiceover] So we're asked, Is the expression x minus three, is this a factor of this fourth degree polynomial? And you could solve this by doing algebraic long division by taking all of this business and dividing it by x minus three and figuring out if you have a remainder. If you do end up with a remainder then this is not a factor of this. But if you don't have a remainder then that means that this divides fully into this right over here without a remainder which means it is a factor. So if the remainder is equal to zero, the remainder is equal to zero, if and only if, it's a factor. It is a factor. And we know a very fast way of calculating the remainder of when you take some polynomial and you divide it by a first degree expression like this. I guess you could say when you divide it by a first degree polynomial like this. The polynomial remainder theorem, the polynomial remainder theorem tells us that if we take some polynomial, p of x and we were to divide it by some x minus a then the remainder is just going to be equal to our polynomial evaluated at our polynomial evaluated at a. So let's just see what's a in this case. Well in this case our a is positive three. So let's just evaluate our polynomial at x equals 3, if what we get is equal to zero that means our remainder is zero and that means that x minus three is a factor. If we get some other remainder that means well we have a non-zero remainder and this isn't a factor, so let's try it out. So, we're gonna have, so I'm just gonna do it all in magenta. It might be a little computationally intensive. So it's going to be two times three to the fourth power, three to the fourth, three to (mumbles), that's 81. 81. Minus 11 Yeah, this is gonna get a little computationally intensive but let's see if we can power through it. 11 times 27, I probably should have picked a simpler example, but let's just keep going. Plus 15 times nine. Plus four times three is 12. Minus 12 So lucky for us, at least those last two terms cancel out. And so this is going to be the rest from here is arithmetic. Two times 81 is 162. Now let's think about what 27 times 11 is. So let's see, 27 times 10 is going to be 270. 270 plus another 27 is minus 297. 297, did I do that, yeah, 270 So 27 times 10 is 270 plus 27, 297 Yep, that's right. And then we have, I'm prone to make careless errors here, see 90 plus 45 is 135. So plus 135. And let's see, if I were to take if I were to take 162 and 135, that's going to give me 297 minus 297. Minus 200, we do it in that green color, minus 297. And we do indeed equal zero. So the remainder, if I were to divide this by this, is equal to zero. So x minus three is indeed a factor of all of this business.