# GRE Subject Test: Math : L'Hospital's Rule

## Example Questions

### Example Question #1 : L'hospital's Rule

Calculate the following limit.

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 #1 : L'hospital's Rule

Find the

.

Does Not Exist

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 #9 : L'hospital's Rule

Evaluate the following limit.

Explanation:

If we plug in 0 into the limit we get , which is indeterminate.

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

.

Now if we plug in 0, we get 0, so that is our final limit.

### Example Question #10 : L'hospital's Rule

Evaluate the following limit

if possible.

Limit does not exist

Explanation:

If we try to directly plug in the limit value into the function, we get

Because the limit is of the form , we can apply L'Hopital's rule to "simplify" the limit to

.

Now if we directly plug in 0 again, we get

.

### Example Question #11 : L'hospital's Rule

Find the limit:

Explanation:

In order to determine the limit, substitute  to determine whether the expression is indeterminate.

Use the L'Hopital's rule to simplify.  Take the derivative of the numerator and denominator separately, and reapply the limit.

Substitute

### Example Question #12 : L'hospital's Rule

Evaluate:

The limit does not exist.

Explanation:

By substitution, the limit will yield an indeterminate form .  L'Hopital can be used in this scenario.

Take the derivative of the numerator and denominator separately, and then reapply the limit.

### Example Question #13 : L'hospital's Rule

Evaluate the following limit:

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 #14 : L'hospital's Rule

Determine the limit of:

Undefined

Explanation:

Rewrite the expression.

By substitutition, we will get the indeterminate form .

The L'Hopital's rule can be used to solve for the limit.  Write the L'Hopital's rule.

Apply this rule twice.

### Example Question #15 : L'hospital's Rule

Evaluate the following limit:

Explanation:

When evaluating the limit using normal methods (substitution), you find that the indeterminate form of  is reached. To evaluate the limit, we can use L'Hopital's Rule, which states that:

So, we find the derivative of the numerator and denominator, which is

and , respectively.

The derivatives were found using the following rules:

When we evaluate this new limit, we find that

.

### Example Question #11 : L'hospital's Rule

Evaluate the limit: