Precise definition of the limit
We say that the limit of as approaches is the number if, for every number , there exists a corresponding number such that whenever . In this interactive figure, you can move the point on the -axis and the point on the -axis. You can change and independently using the sliders or enter itself (even as a formula involving but use it as "e" in that formula). You can also change the function, by using the slider or by entering a function. When proving that the limit of function at is , the goal is to KNOW that for every , there is always a choice of so that if is within of , then is within of ; in other words, if , then . Remember that what happens to at is not important, and that is why is part of the definition.
You can see that a limit exists graphically if for every (which produces the yellow tolerance band around ), you can find (the blue tolerance band around ). The goal is to produce a so that if is within the blue band (), then must also be in the yellow band (). If that is possible, regardless of , the limit is . If that is not always possible, the limit isn't (perhaps the limit attempted is incorrect, or perhaps the limit does not exist).
You can do a bit of zooming by holding the shift key and mouse button while on an axis and dragging to change the scale on that axis. Using the scroll wheel on a mouse will also allow for zooming and you can drag the graph to keep the point of interest near the center of the graph. After zooming, it is probably easiest to reload the interactive figure to start another investigation.
This interactive figure is quite involved.
1. You first enter the function or use the slider to select from given examples.
2. You then adjust and to what seem to be appropriate values to work with.
3. You are now ready to see how and relate. Check the box to show the tolerance band of . You can then experiment some with to see how changes the tolerance band of .
4. You want to see what happens as approaches , so you now need to check the box to show . You can move to see how changes and that sometimes is within the tolerance band of and sometimes is not.
5. Now check to show . As you move , you can see if is within the tolerance band of . Does getting within the tolerance band of force to be in the tolerance band of ? If not, is there a choice of that would accomplish this? You can change using the slider or by entering it. You can use a formula involving , but will have to use e (no easy way to use the keyboard to enter here) and this value of does not update if you vary later.
Developed for use with Thomas' Calculus, published by Pearson.