AP Calculus BC : Applications of Derivatives

Study concepts, example questions & explanations for AP Calculus BC

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Example Questions

Example Question #1 : L'hospital's Rule

Evaluate the limit using L'Hopital's Rule.

Possible Answers:

Undefined

Correct answer:

Explanation:

L'Hopital's Rule is used to evaluate complicated limits. The rule has you take the derivative of both the numerator and denominator individually to simplify the function. In the given function we take the derivatives the first time and get

This still cannot be evaluated properly, so we will take the derivative of both the top and bottom individually again. This time we get

.

Now we have only one x, so we can evaluate when x is infinity. Plug in infinity for x and we get

 and 

So we can simplify the function by remembering that any number divided by infinity gives you zero.

Example Question #1 : L'hospital's Rule

Evaluate the limit using L'Hopital's Rule.

Possible Answers:

Undefined

Correct answer:

Explanation:

L'Hopital's Rule is used to evaluate complicated limits. The rule has you take the derivative of both the numerator and denominator individually to simplify the function. In the given function we take the derivatives the first time and get 

Since the first set of derivatives eliminates an x term, we can plug in zero for the x term that remains. We do this because the limit approaches zero.

This gives us

.

Example Question #21 : Applications Of Derivatives

Evaluate the limit using L'Hopital's Rule.

Possible Answers:

Undefined

Correct answer:

Explanation:

L'Hopital's Rule is used to evaluate complicated limits. The rule has you take the derivative of both the numerator and denominator individually to simplify the function. In the given function we take the derivatives the first time and get 

This still cannot be evaluated properly, so we will take the derivative of both the top and bottom individually again. This time we get

.

Now we have only one x, so we can evaluate when x is infinity. Plug in infinity for x and we get

Example Question #3 : L'hospital's Rule

Calculate the following limit.

Possible Answers:

Correct answer:

Explanation:

To calculate the limit, often times we can just plug in the limit value into the expression. However, in this case if we were to do that we get , which is undefined.

What we can do to fix this is use L'Hopital's rule, which says

.

So, L'Hopital's rule allows us to take the derivative of both the top and the bottom and still obtain the same limit.

.

Plug in  to get an answer of .

Example Question #4 : L'hospital's Rule

Calculate the following limit.

Possible Answers:

Correct answer:

Explanation:

If we plugged in  directly, we would get an indeterminate value of .

We can use L'Hopital's rule to fix this. We take the derivate of the top and bottom and reevaluate the same limit.

.

We still can't evaluate the limit of the new expression, so we do it one more time.

Example Question #22 : Applications Of Derivatives

Find the 

.

Possible Answers:

Does Not Exist

Correct answer:

Explanation:

Subbing in zero into  will give you , so we can try to use L'hopital's Rule to solve.

First, let's find the derivative of the numerator. 

 is in the form , which has the derivative , so its derivative is 

 is in the form , which has the derivative , so its derivative is .

The derivative of  is  so the derivative of the numerator is .

In the denominator, the derivative of  is , and the derivative of  is . Thus, the entire denominator's derivative is .

Now we take the 

, which gives us 

Example Question #11 : L'hospital's Rule

Evaluate the following limit:

Possible Answers:

Correct answer:

Explanation:

When you try to solve the limit using normal methods, you find that the limit approaches zero in the numerator and denominator, resulting in an indeterminate form "0/0". 

In order to evaluate the limit, we must use L'Hopital's Rule, which states that:

when an indeterminate form occurs when evaluting the limit.

Next, simply find f'(x) and g'(x) for this limit:

The derivatives were found using the following rules:

Next, using L'Hopital's Rule, evaluate the limit using f'(x) and g'(x):

Example Question #23 : Applications Of Derivatives

Find the limit if it exists.

Hint: Apply L'Hospital's Rule.

Possible Answers:

Correct answer:

Explanation:

Through direct substitution, we see that the limit becomes

which is in indeterminate form.

As such we can use l'Hospital's Rule, which states that if the limit 

is in indeterminate form, then the limit is equivalent to

Taking the derivatives we use the power rule which states 

Using the power rule the limit becomes 

As such the limit exists and is

Example Question #13 : L'hospital's Rule

Find the limit if it exists.

Hint: Apply L'Hospital's Rule.

Possible Answers:

Correct answer:

Explanation:

Through direct substitution, we see that the limit becomes

which is in indeterminate form.

As such we can use l'Hospital's Rule, which states that if the limit 

is in indeterminate form, then the limit is equivalent to

Taking the derivatives we use the trigonometric rule which states 

 

where  is a constant.

Using l'Hospital's Rule we obtain

And through direct substitution we find

As such the limit exists and is

Example Question #21 : L'hospital's Rule

Find the limit:  

Possible Answers:

Correct answer:

Explanation:

By substituting the value of , we will find that this will give us the indeterminate form .  This means that we can use L'Hopital's rule to solve this problem.

L'Hopital states that we can take the limit of the fraction of the derivative of the numerator over the derivative of the denominator.  L'Hopital's rule can be repeated as long as we have an indeterminate form after every substitution.

Take the derivative of the numerator.

Take the derivative of the numerator.

Rewrite the limit and use substitution.

The limit is .

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