AP Calculus AB : Relationship between differentiability and continuity

Study concepts, example questions & explanations for AP Calculus AB

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Example Questions

Example Question #1 : Relationship Between Differentiability And Continuity

The function  is differentiable at the point . List which of the following statements must be true about :  

 

1)   The limit     exists. 

_________________________________________________________

2)   

_________________________________________________________

3)   

_________________________________________________________

4)   

_________________________________________________________

5)   

 

 

 

Possible Answers:

1, 2, and 4 

1, and 5

1, 3, and 5

1,3,4,and 5

All must be true. 

Correct answer:

1, 2, and 4 

Explanation:

1) If a function is differentiable, then by definition of differentiability the limit defined by, 

  

exists. Therefore (1) is required by definition of differentiability. _______________________________________________________________

2) If a function is differentiable at a point then it must also be continuous at that point. (This is not conversely true).

For a function to be continuous at a point  we must have: 

 

Therefore (2) and (4) are required. 

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3) 

                                 

This is not required, the left side of the equation is the definition of a derivative at a point  for a function . The derivative at a point does not have to equal to the function value  at that point, it is equal to the slope  at that point. Therefore 3 does not have to be true. 

 

However, we can note that it is possible for a function and its' derivative to be equal for a given point. Sine and cosine, for instance will intersect periodically. Another example would be the exponential function  which has itself as its' derivative 

 ______________________________________________________________

 4) See 2

_______________________________________________________________ 

5) 

 

Again, the function does not have to approach the same limit as its' derivative. It is possible for a function to behave in this manner, such as in the case of sine and its' derivative cosine, which will both have the same limit at points where they intersect.  

 

Example Question #1 : Relationship Between Differentiability And Continuity

When the limit  fails to exist,

Possible Answers:

The function  is not continuous at .

The function  is not defined at .

The function  is not differentiable at .

None of the above necessarily

Correct answer:

The function  is not differentiable at .

Explanation:

By definition of differentiability,  when the limit exists. When  exists, we say the function is 'differentiable at '.

Example Question #2 : Relationship Between Differentiability And Continuity

Which of the following functions is differentiable at ,  but not continuous there?

Possible Answers:

They are all differentiable and continuous at 

Correct answer:

They are all differentiable and continuous at 

Explanation:

All of the functions are differentiable at . If you examine the graph of each of the functions, they are all defined at , and do not have a corner, cusp, or a jump there; they are all smooth and connected (Not necessarily everywhere, just at ). Additionally it is not possible to have a function that is differentiable at a point, but not continuous at that same point; differentiablity implies continuity.

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