Two Points Determine a...
The code from the previous activity has created a second point just to the right of 
to create the line between these two points.
If you have trouble selecting the two points, you can set
A on the graph of f(x). The variable h controls this second point by providing a "nudge" to the x coordinate of A. If h is positive, the new point is to the right of A. If h is negative, the new point is to the left. If h is 0, the new point is the same as A.
It's best to think of h as providing a "small nudge" or "small difference" in the x-coordinate of A. Indeed, some calculus books refer to h as Δx ("Delta x"); the use of the Greek letter Δ ("Delta") is meant to invoke the idea of a difference because the word "difference" starts with the letter "d".
Cool.
In my experience, I've found that when mathematicians use Greek letters, more confusion arises than clarity is produced, so I'll avoid using greek characters as best I can, and for now we'll just stick with h.
Use the line tool to create the line between these two points.
If you have trouble selecting the two points, you can set h to a larger value (0.3 for instance) to make it easier to click on them one at a time.
Once the line between the two points is created, move h, and notice that the line changes. You might notice that when h is very close to 0, the slope of the line very closely matches the graph of f(x). If we think of f(x) as "growing" from left to right, you might say that the line matches this growth. More on this in the next chapter.
However, also observe that when you set h to 0, the line disappears! The reason is quite simple: you need two unique points to determine a line, and when h is 0 the two points are not unique.Move forward to see what this mysterious variable
h has to do with the limit concept this chapter is supposed to be all about!