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## AP®︎/College Calculus AB

### Unit 1: Lesson 14

Connecting infinite limits and vertical asymptotes# Infinite limits and asymptotes

AP.CALC:

LIM‑2 (EU)

, LIM‑2.D (LO)

, LIM‑2.D.1 (EK)

, LIM‑2.D.2 (EK)

Unbounded limits are represented graphically by vertical asymptotes and limits at infinity are represented graphically by horizontal asymptotes.

## Want to join the conversation?

- I thought you could never cross asymptotes, only approach; if so then you could definitely cross vertical asymptotes, like the multy-function 3y/(y^2-5)+sin(y)=x(10 votes)
- Here is the confusing thing about asymptotes. You can never cross a vertical asymptote, but you can cross a horizontal or oblique (slant) asymptote. The reason you cannot cross a vertical asymptote is that at the points on the asymptote, the function is undefined because the x value would make the denominator zero. I hope this makes sense!(22 votes)

- Can there be diagonal asymptotes?(7 votes)
- Yes, you will see diagonal asymptotes with hyperbolas, which can be found in the Conic Sections unit in the Precalculus subject:"https://www.khanacademy.org/math/precalculus/conics-precalc".(16 votes)

- The horizontal cases seem a bit counter-intuitive for me. Why do we talk about it here. Are there any applications of the horizontal asymptotes in maths problems or in real life. I just felt a bit confused about graphs with horizontal asymptotes.(2 votes)
- Plenty of applications. A horizontal asymptote can often be interpreted as an upper or lower limit for a problem. For example, if we were to have a logistic function modeling the spread of the coronavirus, the upper horizontal asymptote (limit as x goes to positive infinity) would probably be the size of the Earth's population, since the maximum number of people that can get the virus is the number of people on the planet. If you are not familiar with the shape of a logistic graph please look it up to see.

Hyperbolic functions such as y = 1/x represent an inverse variation relationship between two quantities x and y. Many things in life vary inversely, for example the number of cigarettes someone smokes and their life expectancy. If you smoke 10 packs a day, your life expectancy will significantly decrease. The horizontal asymptote represents the idea that if you were to smoke more and more packs of cigarettes, your life expectancy would be decreasing. If it made sense to smoke infinite cigarettes, your life expectancy would be zero.(10 votes)

- How can we distinguish a horizontal and vertical asymptote in higher dimensions? Don't they loose meaning? Since everything is according to once point of reference (this might be the key, BUT) Does that mean that for every object in space we asign a coordinate plane to determine their point and asymptotes?

Sorry if I cannot get to explain myself or this may sound stupid, so many questions.(3 votes)- We tend not to use the terms 'horizontal/vertical asymptote' in more than 2 dimensions. If a curve or surface was asymptotic to some line or plane, we would care more about finding the equation of that line or plane, which tells us everything we want to know about it, than about whether to call it horizontal or vertical.(1 vote)

- why wont a vertical asymptote be crossed ? ( said in3:39)(3 votes)
- i do not understand what are asymptotes. Is there nother video that explains it or can someone help ?

thank you(2 votes)- An asymptote is when the line approaches an x or y value, but does not reach it.

To get a visual on this topic, I would plug the equation y=1/x into a graphing calculator. The asymptotes that you will see are x=0, (the line soars up to infinity on one side, and down to negative infinity on the other), and y=0, (as x goes to infinity, the line gets closer and closer to the x-axis, but it never touches).

I also tried to find a video on this topic, but I couldn't find one, so I hope my explanation helps you out.(2 votes)

- If I want to get limit of f(x) as x approaches to infinity, how do we get that limit from positive side? It seemed that if this works, there has to have something bigger than infinity(1 vote)
- In the next section, "limits at infinity", You can learn all you need to find a limit at infinity.

https://www.khanacademy.org/math/differential-calculus/dc-limits/dc-limits-at-infinity/v/introduction-to-limits-at-infinity(3 votes)

- That wavy transformation using sin(x) was pretty cool.(2 votes)
- If the limit is undefined is it necessary to show the working(1 vote)
- Yes, you have to show that the limit is undefined. You have to show work in all cases in all problems. Just writing "Undefined" on your homework is not a quick way out :-)(1 vote)

- How could these be used/represented in real life?(1 vote)

## Video transcript

- [Instructor] What we're
going to do in this video is use the online graphing
calculator Desmos, and explore the relationship
between vertical and horizontal asymptotes, and think about how they relate to what we know about limits. So let's first graph two over x minus one, so let me get that one graphed, and so you can immediately
see that something interesting happens at x is equal to one. If you were to just
substitute x equals one into this expression, you're going to get two over zero, and whenever
you get a non-zero thing, over zero, that's a
good sign that you might be dealing with a vertical asymptote. In fact we can draw
that vertical asymptote right over here at x equals one. But let's think about how
that relates to limits. What if we were to explore
the limit as x approaches one of f of x is equal to
two over x minus one, and we could think about it from the left and from the right, so if we approach one from the left, let me zoom in a little bit over here, so we can see as
we approach from the left when x is equal to zero, the f of x would be equal to negative two, when x is equal to point five, f of x is equal to negative of
four, and then it just gets more and more negative the closer we get to one from the left. I could really, so I'm
not even that close yet if I get to let's say 0.91,
I'm still nine hundredths less than one, I'm at
negative 22.222, already. And so the limit as we
approach one from the left is unbounded, some people would say it goes to negative
infinity, but it's really an undefined limit, it is unbounded in the negative direction. And likewise, as we
approach from the right, we get unbounded in the
positive infinity direction and technically we would
say that that limit does not exist. And this would be the
case when we're dealing with a vertical asymptote
like we see over here. Now let's compare that
to a horizontal asymptote where it turns out that the limit actually can exist. So let me delete these or
just erase them for now, and so let's look at this function which is a pretty neat
function, I made it up right before this video started but it's kind of cool
looking, but let's think about the behavior as
x approaches infinity. So as x approaches infinity,
it looks like our y value or the value of the
expression, if we said y is equal to that expression, it looks like it's getting closer and
closer and closer to three. And so we could say that we
have a horizontal asymptote at y is equal to three, and we could also and there's a more rigorous
way of defining it, say that our limit as
x approaches infinity is equal of the expression
or of the function, is equal to three. Notice my mouse is
covering it a little bit as we get larger and larger, we're getting closer and closer to three, in fact we're getting
so close now, well here you can see we're
getting closer and closer and closer to three. And you could also
think about what happens as x approaches negative infinity and here you're getting closer
and closer and closer to three from below. Now one thing that's
interesting about horizontal asymptotes is you might
see that the function actually can cross a horizontal asymptote. It's crossing this horizontal asymptote in this area in between and
even as we approach infinity or negative infinity, you can oscillate around that horizontal asymptote. Let me set this up, let me
multiply this times sine of x. And so there you have it,
we are now oscillating around the horizontal asymptote, and once again this limit can exist even though we keep crossing
the horizontal asymptote, we're getting closer and
closer and closer to it the larger x gets. And that's actually the
key difference between a horizontal and a vertical asymptote. Vertical asymptotes if you're
dealing with a function, you're not going to cross
it, while with a horizontal asymptote, you could,
and you are just getting closer and closer and closer to it as x goes to positive infinity or as x goes to negative infinity.