AP Calculus AB : Derivatives

Study concepts, example questions & explanations for AP Calculus AB

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Example Questions

Example Question #72 : Derivative As A Function

At the point , is the function  increasing or decreasing, concave or convex?

Possible Answers:

Increasing, concave

Increasing, convex

Decreasing, convex

The function is undefined at that point

Decreasing, concave

Correct answer:

Decreasing, convex

Explanation:

First, let's find out if the graph is increasing or decreasing. For that, we need the first derivative.

To find the first derivative, we can use the power rule. We lower the exponent on all the variables by one and multiply by the original variable.

We're going to treat  as  since anything to the zero power is one.

Notice that  since anything times zero is zero.

Plug in our given point for . If the result is positive, the function is increasing. If the result is negative, the function is decreasing.

Our result is negative, therefore the function is decreasing.

To find the concavity, look at the second derivative. If the function is positive at our given point, it is concave. If the function is negative, it is convex.

To find the second derivative we repeat the process, but using  as our expression.

As you can see, our second derivative is a constant. It doesn't matter what point we plug in for ; our output will always be negative. Therefore our graph will always be convex.

Combine our two pieces of information to see that at the given point, the graph is decreasing and convex.

Example Question #3 : Finding Regions Of Concavity And Convexity

When , what is the concavity of the graph of ?

Possible Answers:

Increasing, concave

Decreasing, concave

Decreasing, convex

There is insufficient data to solve.

Increasing, convex

Correct answer:

Increasing, convex

Explanation:

To find the concavity, we need to look at the first and second derivatives at the given point. 

To take the first derivative of this equation, use the power rule. The power rule says that we lower the exponent of each variable by one and multiply that number by the original exponent:

Simplify:

Remember that anything to the zero power is equal to one.

The first derivative tells us if the function is increasing or decreasing. Plug in the given point, , to see if the result is positive (i.e. increasing) or negative (i.e. decreasing).

Therefore the function is increasing.

To find out if the function is convex, we need to look at the second derivative evaluated at the same point, , and check if it is positive or negative.

We're going to treat  as  since anything to the zero power is equal to one.

Notice that  since anything times zero is zero.

Plug in our given value:

Since the second derivative is positive, the function is convex. 

Therefore, we are looking at a graph that is both increasing and convex at our given point.

Example Question #2 : Finding Regions Of Concavity And Convexity

At the point , is  increasing or decreasing, and is it concave or convex?

Possible Answers:

Increasing, convex

Decreasing, concave

Increasing, concave

The graph is undefined at point 

Decreasing, convex

Correct answer:

Decreasing, convex

Explanation:

To find out if the function is increasing or decreasing, we need to look at the first derivative.

To find the first derivative, we can use the power rule. We lower the exponent on all the variables by one and multiply by the original variable.

Anything to the zero power is one.

Now we plug in our given value and find out if the result is positive or negative. If it is positive, the function is increasing. If it is negative, the function is decreasing.

Therefore, the function is decreasing.

To find out if it is concave or convex, look at the second derivative. If the result is positive, it is convex. If it is negative, then it is concave.

To find the second derivative, we repeat the process using  as our expression.

We're going to treat  as .

Notice that  since anything times zero is zero.

As stated before, anything to the zero power is one.

Since we get a positive constant, it doesn't matter where we look on the graph, as our second derivative will always be positive. That means that this graph is going to be convex at our given point.

Therefore, the function is decreasing and convex at our given point.

Example Question #12 : Calculus I — Derivatives

Define .

Give the interval(s) on which  is decreasing.

Possible Answers:

Correct answer:

Explanation:

 is decreasing on those intervals at which .

 

 

We need to find the values of  for which . To that end, we first solve the equation:

 

These are the boundary points, so the intervals we need to check are:

,  and 

 

We check each interval by substituting an arbitrary value from each for .

 

Choose 

 increases on this interval.

 

Choose 

 decreases on this interval.

 

Choose 

 increases on this interval.

 

The answer is that  decreases on .

Example Question #13 : Calculus I — Derivatives

Define .

Give the interval(s) on which  is increasing.

Possible Answers:

Correct answer:

Explanation:

 is increasing on those intervals at which .

 

 

We need to find the values of  for which . To that end, we first solve the equation:

 

These are the boundary points, so the intervals we need to check are:

,  and 

We check each interval by substituting an arbitrary value from each for .

 

Choose 

 increases on this interval.

 

Choose 

 decreases on this interval.

 

Choose 

 increases on this interval.

 

The answer is that  increases on 

Example Question #3 : Derivatives

At what point does  shift from increasing to decreasing?

Possible Answers:

It does not shift from increasing to decreasing

Correct answer:

Explanation:

To find out where the graph shifts from increasing to decreasing, we need to look at the first derivative. 

To find the first derivative, we can use the power rule. We lower the exponent on all the variables by one and multiply by the original variable.

We're going to treat  as  since anything to the zero power is one.

Notice that  since anything times zero is zero.

If we were to graph , would the y-value change from positive to negative? Yes. Plug in zero for y and solve for x.

Example Question #1 : Derivatives

At what value of  does  shift from decreasing to increasing?

Possible Answers:

It does not shift from decreasing to increasing

Correct answer:

Explanation:

To find out when the function shifts from decreasing to increasing, we look at the first derivative.

To find the first derivative, we can use the power rule. We lower the exponent on all the variables by one and multiply by the original variable.

Anything to the zero power is one.

From here, we want to know if there is a point at which graph changes from negative to positive. Plug in zero for y and solve for x.

This is the point where the graph shifts from decreasing to increasing.

 

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

Evaluate the following limit:

Possible Answers:

Correct answer:

Explanation:

When  approaches 0 both  and  will approach . Therefore, L’Hopital’s Rule can be applied here. Take the derivatives of the numerator and denominator and try the limit again:

 

Example Question #2 : Derivative Interpreted As An Instantaneous Rate Of Change

Find the instantaneous rate of change for the function, 

at the point 

Possible Answers:

Correct answer:

Explanation:

Find the instantaneous rate of change for the function, 

at the point 

 

1) First compute the derivative of the function, since this will give us the instantaneous rate of change of the function as a function of 

 

 

 

2) Now evaluate the derivative at the value 

 

Therefore,  is the instantaneous rate of change of the function 

 at the point 

 

Example Question #3 : Derivative Interpreted As An Instantaneous Rate Of Change

A particle is traveling in a straight line along the x-axis with position function . What is the instantaneous rate of change in the particle's position at time  seconds?

Possible Answers:

Correct answer:

Explanation:

To find the instantaneous rate of change of the particle at time , we have to find the derivative of  and plug  into it.

.

And

.

Hence the instantaneous rate of change in position (or just 'velocity') of the particle at  is . (At that very instant, the particle is not moving.) 

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