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Limit properties

This video introduces limit properties, which are intuitive rules that help simplify limit problems. The main properties covered are the sum, difference, product, quotient, and exponent rules. These properties allow you to break down complex limits into simpler components, making it easier to find the limit of a function. Created by Sal Khan.

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  • leaf green style avatar for user Themis
    I seen in my book the notation like lim f(x+y). Is this the same as f(x)+f(y) ?
    Also my teacher said that ln(x+y) is not lnx + lny but ln(x,y) = ln(x)+ln(y). Does ln represent a function?
    (87 votes)
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  • blobby green style avatar for user Bryan Glesmann
    What I do not understand about limits is why you would want to do something like multiply or add them? How can limits be related to each other, let alone multiplied? Aren't limits simply in respect to themselves as a limit, and not in respect to other limits?
    (66 votes)
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    • leaf blue style avatar for user Matthew Daly
      They can be. For instance, let's say that I had a rectangle made out of metal that expanded or contracted depending on the temperature. If I knew that the length of the rectangle approached 3 meters as the temperature approached 35 degrees and the width of the rectangle approached 4 meters as the temperature approached 35 degrees. Then it would be useful and sensible for me to conclude that the area of the rectangle would approach 12 square meters as the temperature approached 35 degrees.
      (225 votes)
  • piceratops ultimate style avatar for user grantgilmore6
    At , note that the lim of g(x) as it approaches c can not be 0. If it is, then the entire limit would not exist because a number divided by 0 is undefined.
    (13 votes)
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    • piceratops ultimate style avatar for user Just Keith
      This is an excellent question because it is a point that beginners are often confused about.

      You are thinking of limits incorrectly. A limit is NOT what you would get if you actually did the math of the expression at the limiting value. The limit is what you would be approaching as you got extremely close to, but not equal to, the limiting value.

      The whole point in bothering with limits is finding ways of getting values that you cannot directly compute (usually division by 0 or other undefined or indeterminate forms).

      Thus, lim x→0 1/x² = infinity

      You would not plug in x = 0, you would examine what happens when you get extremely close to x=0. For example, what is 1/x² when x = 1×10⁻¹²³? It is 10²⁴⁶. So, as you get closer and closer to x=0, clearly this is heading toward infinity.

      Now, it is the case that IF and ONLY IF the expression is both defined and continuous at the limiting value, then the limit can be found just by plugging in the limiting value. However, if the expression is either not continuous or not defined at that point, then you must use other means of finding the limit.
      (39 votes)
  • starky ultimate style avatar for user mr.swedishfish
    Where can I find the "rigorous proof" of these properties?
    (14 votes)
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    • mr pants teal style avatar for user Moon Bears
      A rigorous proof can usually be found in any old calculus text, in the section on limits. A fun exercise might be to write down the epsilon-delta definition of limits then try to figure out exactly how one would prove these statements!
      (11 votes)
  • starky seedling style avatar for user Cooper Maruyama
    At you wrote:
    lim f(x)g(x) = lim f(x) * g(x)
    This leads me to understand that lim ab = lim a * lim b

    However, at , you wrote:
    lim kf(x) = k * lim f(x)

    If lim ab = lim a * lim b, shouldn't lim kf(x) = lim k * lim f(x)?
    (5 votes)
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  • leaf green style avatar for user popcorn.tomato.dude
    for the exponent property, why assume that the power is a rational number? would this still work if the power was sqrt(3) or something like that?
    (6 votes)
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  • aqualine ultimate style avatar for user Bianca Otake
    Concerning , If you get the question "what is the product of the limit g(x) when x approaches c and the limit f(x) when x approaches c" given that f(x) is discontinuous at c. Would the answer be zero or just that the limit does not exist?
    (3 votes)
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  • aqualine ultimate style avatar for user V22688622V
    5 videos in a row?!
    (6 votes)
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  • blobby green style avatar for user Montana Burr
    What is the limit of f(x) raised to the g(x) power?
    (4 votes)
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  • blobby green style avatar for user Keen
    What am I supposed to do if the limit of the 2 functions are different?
    (2 votes)
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    • mr pink red style avatar for user Forever Learner
      what do ya mean? it doesnt matter if the limit of the two functions are different. u just perform the operation that is being asked. Let's say the question is: what's the limit as x approaches four of f(x)+g(x).
      So you find on the graph that as x approaches four, f(x) approaches 7 while g(X) approaches 5. Because the original question asks you to add those two limits together, you get 12. thus, the limit as x approaches four of f(x)+g(x) in my hypothetical question is 12.
      (6 votes)

Video transcript

What I want to do in this video is give you a bunch of properties of limits. And we're not going to prove it rigorously here. In order to have the rigorous proof of these properties, we need a rigorous definition of what a limit is. And we're not doing that in this tutorial, we'll do that in the tutorial on the epsilon delta definition of limits. But most of these should be fairly intuitive. And they are very helpful for simplifying limit problems in the future. So let's say we know that the limit of some function f of x, as x approaches c, is equal to capital L. And let's say that we also know that the limit of some other function, let's say g of x, as x approaches c, is equal to capital M. Now given that, what would be the limit of f of x plus g of x as x approaches c? Well-- and you could look at this visually, if you look at the graphs of two arbitrary functions, you would essentially just add those two functions-- it'll be pretty clear that this is going to be equal to-- and once again, I'm not doing a rigorous proof, I'm just really giving you the properties here-- this is going to be the limit of f of x as x approaches c, plus the limit of g of x as x approaches c. Which is equal to, well this right over here is-- let me do that in that same color-- this right here is just equal to L. It's going to be equal to L plus M. This right over here is equal to M. Not too difficult. This is often called the sum rule, or the sum property, of limits. And we could come up with a very similar one with differences. The limit as x approaches c of f of x minus g of x, is just going to be L minus M. It's just the limit of f of x as x approaches c, minus the limit of g of x as x approaches c. So it's just going to be L minus M. And we also often call it the difference rule, or the difference property, of limits. And these once again, are very, very, hopefully, reasonably intuitive. Now what happens if you take the product of the functions? The limit of f of x times g of x as x approaches c. Well lucky for us, this is going to be equal to the limit of f of x as x approaches c, times the limit of g of x, as x approaches c. Lucky for us, this is kind of a fairly intuitive property of limits. So in this case, this is just going to be equal to, this is L times M. This is just going to be L times M. Same thing, if instead of having a function here, we had a constant. So if we just had the limit-- let me do it in that same color-- the limit of k times f of x, as x approaches c, where k is just some constant. This is going to be the same thing as k times the limit of f of x as x approaches c. And that is just equal to L. So this whole thing simplifies to k times L. And we can do the same thing with difference. This is often called the constant multiple property. We can do the same thing with differences. So if we have the limit as x approaches c of f of x divided by g of x. This is the exact same thing as the limit of f of x as x approaches c, divided by the limit of g of x as x approaches c. Which is going to be equal to-- I think you get it now-- this is going to be equal to L over M. And finally-- this is sometimes called the quotient property-- finally we'll look at the exponent property. So if I have the limit of-- let me write it this way-- of f of x to some power. And actually, let me even write it as a fractional power, to the r over s power, where both r and s are integers, then the limit of f of x to the r over s power as x approaches c, is going to be the exact same thing as the limit of f of x as x approaches c raised to the r over s power. Once again, when r and s are both integers, and s is not equal to 0. Otherwise this exponent would not make much sense. And this is the same thing as L to the r over s power. So this is equal to L to the r over s power. So using these, we can actually find the limit of many, many, many things. And what's neat about it is the property of limits kind of are the things that you would naturally want to do. And if you graph some of these functions, it actually turns out to be quite intuitive.