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  • - [Instructor] The goal of this video

  • is to get an appreciation that you could have many,

  • in fact, you could have an infinite number

  • of functions that have the same limit

  • as x approaches infinity.

  • So, if we were to make the general statement

  • that the limit of some function f of x,

  • as x approaches infinity, is equal to three.

  • What I wanna do in this video

  • is show some examples of that.

  • And to show that we can keep creating

  • more and more examples, really an infinite number

  • of examples where that is going to be true.

  • So, for example, we could look at this graph over here.

  • And in other videos, we'll think about why this is the case,

  • but just think about what happens when

  • you have very, very large Xs.

  • When you have very, very large Xs,

  • the plus five doesn't matter as much,

  • and so it gets closer and closer to three x squared

  • over x squared, which is equal to three.

  • And you could see that right over here,

  • it's graphed in this green color.

  • And you can see, even when x is equal to 10,

  • we're getting awfully close to three right over there.

  • Let me zoom out a little bit so you see our axes.

  • So that is three.

  • Let me draw a dotted line at the asymptote.

  • That is y is equal to three, and so you see

  • the function's getting closer and closer

  • as x approaches infinity.

  • But that's not the only function that could do that,

  • as I keep saying, there's an infinite number

  • of functions that could do that.

  • You could have this somewhat wild function

  • that involves natural logs.

  • That too, as x approaches infinity,

  • it is getting closer and closer to three.

  • It might be getting closer to three

  • at a slightly slower rate than the one in green,

  • but we're talking about infinity.

  • As x approaches infinity, this thing is approaching three.

  • And as we've talked about in other videos,

  • you could even have things that keep

  • oscillating around the asymptote,

  • as long as they're getting closer and closer and closer

  • to it as x gets larger and larger and larger.

  • So, for example, that function right over there.

  • Let me zoom in.

  • So, let's zoom in.

  • Let's say when x is equal to 14,

  • we can see that they're all approaching three.

  • The purple one is oscillating around it,

  • the other two are approaching three from below.

  • But as we get much larger,

  • let me actually zoom out a ways, and then I'll zoom in.

  • So let's get to really large values.

  • So, actually, even 100 isn't even that large

  • if we're thinking about infinity.

  • Even a trillion wouldn't be that large

  • if we're thinking about infinity.

  • But let's go to 200.

  • 200 is much larger than numbers we've been looking at.

  • And let me zoom in

  • when x is equal to 200, and you can see,

  • we have to zoom in an awfully lot, an awful lot,

  • just to even see that the graphs still aren't quite

  • stabilized around the asymptote,

  • that they are a little bit different than the asymptote.

  • I really zoomed in, I mean look at the scale.

  • This is, each of these are now 100th, each square.

  • And so we've gotten much, much,

  • much closer to the asymptote.

  • In fact, the green function,

  • we still can't tell the difference.

  • You can see the calculation, this is up to

  • three or four decimal places,

  • we're getting awfully close to three now,

  • but we aren't there.

  • So the green functions got there the fastest,

  • is an argument.

  • But the whole point of this is to emphasize the fact

  • that there's an infinite number of functions

  • for which you could make the statement that we made.

  • That the limit of the function as x approaches infinity,

  • in this case, we said that limit is going

  • to be equal to three, and I just picked three arbitrarily.

  • This could be true for any, for any function.

  • I didn't realize how much I had zoomed in.

  • So let me now go back to the origin

  • where we had our original expression.

  • So, there we have it, and maybe I can zoom in this way.

  • So there you have it.

  • Limit of any of these, as x approaches infinity,

  • is equal to three.

- [Instructor] The goal of this video

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B1 中級 美國腔

14-2 無窮大相同極限的函數(Functions with same limit at infinity | Limits and continuity | AP Calculus AB | Khan Academy)

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    yukang920108 發佈於 2022 年 07 月 05 日
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