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## Limits at infinity

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# Limits at infinity of quotients with square roots (even power)

AP Calc: LIM‑2 (EU), LIM‑2.D (LO), LIM‑2.D.3 (EK), LIM‑2.D.4 (EK), LIM‑2.D.5 (EK)

## Video transcript

- [Voiceover] Let's see
if we can find the limit as x approaches negative infinity of the square root of
4x to the fourth minus x over 2x squared plus three. And like always, pause this video and see if you can figure it out. Well, whenever we're trying to find limits at either positive or negative infinity of rational expressions like this, it's useful to look at what
is the highest degree term in the numerator or in the denominator, or, actually in the numerator
and the denominator, and then divide the
numerator and the denominator by that highest degree,
by x to that degree. Because if we do that,
then we're going to end up with some constants and some other things that will go to zero
as we approach positive or negative infinity,
and we should be able to find this limit. So what I'm talking about,
let's divide the numerator by one over x squared and
let's divide the denominator by one over x squared. Now, you might be saying, "Wait, wait, "I see an x to the fourth here. "That's a higher degree." But remember, it's under the radical here. So if you wanna look at
it at a very high level, you're saying, okay, well x
to the fourth, but it's under, you're gonna take the square
root of this entire expression, so you can really view this
as a second degree term. So the highest degree
is really second degree, so let's divide the numerator and the denominator by x squared. And if we do that, dividing, so this is going to be the same thing as, so this is going to be the limit, the limit as x approaches
negative infinity of, so let me just do a
little bit of a side here. So if I have, if I have one over x squared, all right, let me write it. Let me just, one over x
squared times the square root of 4x to the fourth minus x, like we have in the numerator here. This is equal to, this is the same thing as one over the square
root of x to the fourth times the square root of
4x to the fourth minus x. And so this is equal to the square root of 4x to the fourth minus x over x to the fourth, which is
equal to the square root of, and all I did is I brought
the radical in here. You could view this as the
square root of all this divided by the square root of this, which is equal to, just
using our exponent rules, the square root of 4x
to the fourth minus x over x to the fourth. And then this is the
same thing as four minus, x over x to the fourth is
one over x to the third. So this numerator is going to be, the numerator's going
to be the square root of four minus one, x to the third power. And then the denominator is going to be equal to, well, you divide 2x squared by x squared. You're just going to be left with two. And then three divided
by x squared is gonna be three over x squared. Now, let's think about the limit as we approach negative infinity. As we approach negative infinity, this is going to approach zero. One divided by things that are becoming more and more and more and
more and more negative, their magnitude is getting larger, so this is going to approach zero. This over here is also going to be, this thing is also going
to be approaching zero. We're dividing by larger and
larger and larger values. And so what this is going to result in is the square root of four,
the principal root of four, over two, which is the same thing as two over two, which is equal to one. And we are done.