Continuity and differentiability over a closed interval

In summary: The idea is that there might be a point in between p and q where f'(p)>f'(q). But to figure out where this point is, you need to know what f'(p) and f'(q) are. But you can't do that without knowing what c and d are.So in summary, the author is saying that there exists a well-defined maximum value for f, and that this maximum value can only be achieved by a function that is linear and does not have any jumps or sharp edges. If you want to reason about this without any mathematical proofs, I suggest you draw a picture to help visualise what's happening.
  • #1
budafeet57
24
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Homework Statement


http://i.imgur.com/69BmR.jpg

Homework Equations


The Attempt at a Solution


a, c are right because f(c) is continuous.
b, d are right because f'(c) is differentiable over the interval
I am not sure about e. Can anyone explain to me?
 
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  • #2
budafeet57 said:
b, d are right because f'(c) is differentiable over the interval

Are you sure about that? What about a linear function? Then f'(x) = constant ≠ 0.

budafeet57 said:
I am not sure about e. Can anyone explain to me?
What the claim is saying is that there exists a well-defined maximum value for f. How would the function f need to behave for this claim not to be true?
 
  • #3
You appear to have "cut off" a critical part: what is f(1)?
 
  • #4
clamtrox said:
Are you sure about that? What about a linear function? Then f'(x) = constant ≠ 0.What the claim is saying is that there exists a well-defined maximum value for f. How would the function f need to behave for this claim not to be true?
Ah I see, b is the answer. Because f is not a constan, so it can only be linear. When it's differentiated it can't equal to zero under [-2,1].
 
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  • #5
f(1)=4
 
  • #6
budafeet57 said:
Ah I see, b is the answer. Because f is not a constan, so it can only be linear. When it's differentiated it can't equal to zero under [-2,1].

That makes no sense at all. Why won't you draw a picture to get some idea of what's happening, if you want to reason like this without any mathematical proofs.
 
  • #7
clamtrox said:
That makes no sense at all. Why won't you draw a picture to get some idea of what's happening, if you want to reason like this without any mathematical proofs.

Thank you clamtrox. I am very rusty about the definition of continuity and differentiability, even I went back to my calc textbook, I still cannot figure out what I should do. Can you tell me what's wrong with my reasoning?
 
  • #8
I personally think drawing the picture is more informative

I think the problem in your reasoning is that you don't fully understand the problem. So visually, these things you know about the function:
-it starts from p=(-2,-5) and ends in q=(1,4)
-there are no jumps
-there are no sharp edges
You are asked to check if there exists any such function for which the conditions A-E do not hold.

A is easy: you cannot draw a curve from p to q without crossing f=0. This can be shown easily using intermediate value theorem.

Now for B: here you need to check if all curves from p to q must have f'=0 at some point. In otherwords, they are parallel to the x-axis. Now I already gave you one counterexample for why this isn't true. I am sure you can draw several other curves that also are not parallel with x-axis at any points, but still satisfy the other conditions.

Then D is the most interesting one: you should really think hard about this.
 

FAQ: Continuity and differentiability over a closed interval

1. What does it mean for a function to be continuous over a closed interval?

A function is continuous over a closed interval if it is defined and has no breaks or gaps in its graph over that interval. This means that the function can be drawn without lifting the pen from the paper.

2. How can we determine if a function is continuous over a closed interval?

A function is continuous over a closed interval if it satisfies the three conditions of continuity: 1) the function is defined at every point in the interval, 2) the limit of the function as x approaches a point within the interval exists, and 3) the value of the function at that point is equal to the limit. If any of these conditions are not met, the function is not continuous over the closed interval.

3. What is the difference between continuity and differentiability over a closed interval?

Continuity refers to the smoothness of a function over an interval, while differentiability refers to the existence of a derivative at every point in that interval. A function can be continuous but not differentiable at certain points, and vice versa.

4. Can a function be continuous but not differentiable over a closed interval?

Yes, a function can be continuous but not differentiable at certain points within a closed interval. This usually occurs when there is a sharp turn or corner in the graph of the function, as the derivative does not exist at these points.

5. How can we prove that a function is differentiable over a closed interval?

In order to prove that a function is differentiable over a closed interval, we must show that the derivative exists at every point within the interval. This can be done by using the definition of the derivative, taking the limit as h approaches 0 of the difference quotient, and showing that the limit exists and is the same for every point in the interval.

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