Subtitles section Play video Print subtitles - [Instructor] Multiple videos and exercises we cover the various techniques for finding limits. But sometimes, it's helpful to think about strategies for determining which technique to use. And that's what we're going to cover in this video. What you see here is a flowchart developed by the team at Khan Academy, and I'm essentially going to work through that flowchart. It looks a little bit complicated at first, but hopefully it will make sense as we talk it through. So the goal is, hey, we want to find the limit of f of x as x approaches a. So what this is telling us to do is, well the first thing, just try to substitute what happens when x equals a. Let's evaluate f of a. And this flowchart says, if f of a is equal to a real number, it's saying we're done. But then there's this little caveat here. Probably. And the reason why is that the limit is a different thing than the value of the function. Sometimes they happen to be the same. In fact, that's the definition of a continuous function which we talk about in previous videos, but sometimes, they aren't the same. This will not necessarily be true if you're dealing with some function that has a point discontinuity like that or a jump discontinuity, or a function that looks like this. This would not necessarily be the case. But if at that point you're trying to find the limit towards, as you approach this point right over here, the function is continuous, it's behaving somewhat normally, then this is a good thing to keep in mind. You could just say, hey, can I just evaluate the function at that at that a over there? So in general, if you're dealing with pretty plain vanilla functions like an x squared or if you're dealing with rational expressions like this or trigonometric expressions, and if you're able to just evaluate the function and it gives you a real number, you are probably done. If you're dealing with some type of a function that has all sorts of special cases and it's piecewise defined as we've seen in previous other videos, I would be a little bit more skeptical. Or if you know visually around that point, there's some type of jump or some type of discontinuity, you've got to be a little bit more careful. But in general, this is a pretty good rule of thumb. If you're dealing with plain vanilla functions that are continuous, if you evaluate at x equals a and you got a real number, that's probably going to be the limit. But I always think about the other scenarios. What happens if you evaluate it and you get some number divided by zero? Well, that case, you are probably dealing with a vertical asymptote. And what do we mean by vertical asymptote? Well, look at this example right over here. Where we just say the limit put that in a darker color. So if we're talking about the limit as x approaches one of one over x minus one, if you just try to evaluate this expression at x equals one, you would get one over one minus one which is equal to one over zero. It says, okay, I'm throwing it, I'm falling into this vertical asymptote case. And at that point, if you wanted to just understand what was going on there or even verify that it's a vertical asymptote, well then you can try out some numbers, you can try to plot it, you can say, alright, I probably have a vertical asymptote here at x equals one. So that's my vertical asymptote. And you can try out some values. Well, let's see. If x is greater than one, the denominator is going to be positive, and so, my graph and you would get this from trying out a bunch of values. Might look something like this and then for values less than negative one or less than one I should say, you're gonna get negative values and so, your graph might look like something like that until you have this vertical asymptote. That's probably what you have. Now, there are cases, very special cases, where you won't necessarily have the vertical asymptote. One example of that would be something like one over x minus x. This one here is actually undefined for any x you give it. So, it would be very, you will not have a vertical asymptote. But this is a very special case. Most times, you do have a vertical asymptote there. But let's say we don't fall into either of those situations. What if when we evaluate the function, we get zero over zero? And here is an example of that. Limit is x approaches negative one of this rational expression. Let's try to evaluate it. You get negative one squared which is one minus negative one which is plus one minus two. So you get zero the numerator. And the denominator you have negative one squared which is one minus two times negative one so plus two minus three which is equal to zero. Now this is known as indeterminate form. And so on our flowchart, we then continue to the right side of it and so here's a bunch of techniques for trying to tackle something in indeterminate form. And likely in a few weeks you will learn another technique that involves a little more calculus called L'hospital's Rule that we don't tackle here because that involves calculus while all of these techniques can be done with things before calculus. Some algebraic techniques and some trigonometric techniques. So the first thing that you might want to try to do especially if you're dealing with a rational expression like this and you're getting indeterminate form, is try to factor it. Try to see if you can simplify this expression. And this expression here, you can factor it. This is the same thing as x x minus two times x plus one over x this would be x minus three times x plus one if what I just did seems completely foreign to you I encourage you to watch the videos on factoring factoring polynomials