# Calculus 1 : How to find concave down intervals by graphing functions

## Example Questions

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### Example Question #1 : How To Find Concave Down Intervals By Graphing Functions

Find the interval(s) where the following function is concave down. Graph to double check your answer.

Never

Always

Explanation:

To find when a function is concave, you must first take the 2nd derivative, then set it equal to 0, and then find between which zero values the function is negative.

First, find the 2nd derivative:

Set equal to 0 and solve:

Now test values on all sides of these to find when the function is negative, and therefore decreasing. I will test the values of -3 and 0.

Since the value that is negative is when x=-3, the interval is decreasing on the interval that includes 0. Therefore, our answer is:

### Example Question #2 : How To Find Concave Down Intervals By Graphing Functions

Find the interval(s) where the following function is concave down. Graph to double check your answer.

Always

Never

Explanation:

To find when a function is concave, you must first take the 2nd derivative, then set it equal to 0, and then find between which zero values the function is negative.

First, find the 2nd derivative:

Set equal to 0 and solve:

Now test values on all sides of these to find when the function is negative, and therefore decreasing. I will test the values of 0 and 6.

Since the value that is negative is when x=0, the interval is decreasing on the interval that includes 0. Therefore, our answer is:

### Example Question #1 : How To Find Concave Down Intervals By Graphing Functions

Is  concave down on the interval ?

No,  is negative on the interval .

Yes,  is negative on the interval .

No,  is positive on the interval .

Cannot be determined by the information given

Yes,  is positive on the interval .

Yes,  is negative on the interval .

Explanation:

To test concavity, we must first find the second derivative of f(x)

This function is concave down anywhere that f''(x)<0, so...

So,

for all

So on the interval -5,-4 f(x) is concave down because f''(x) is negative.

### Example Question #3 : How To Find Concave Down Intervals By Graphing Functions

Is  is concave up or concave down on the interval ? How do you know?

Concave up, because  is positive on the given interval.

Concave up, because  is positive on the given interval.

Concave down, because  is negative on the given interval.

Concave down, because  is negative on the given interval.

is neither concave up nor concave down on the given interval.

Concave up, because  is positive on the given interval.

Explanation:

To test concavity, we need to perform the second derivative test. Bascially, assuming that our function is twice-differentiable, anywhere that h"(t) is positive will be concave up, and anywhere h"(t) is negative is concave down. Begin as follows:

Next, we need to evaluate h"(t) on the interval [5,7]

So, our h"(t) is positive on the interval, and therefore h(t) is concave up.

### Example Question #1 : How To Find Concave Down Intervals By Graphing Functions

Find the intervals where  is concave up.

Explanation:

The intervals where a function is concave up or down is found by taking second derivative of the function. Use the power rule which states:

Now, set  equal to  to find the point(s) of infleciton.

In this case, .

To find the concave up region, find where  is positive. This will either be to the  left of   or to the right of . To find out which, plug in a test point in each of those regions.

If we plug in  we get , which is negative, so that cannot be concave up.

If we plug in , we get , which is positive, so we know that the region  will be concave up.

### Example Question #6 : How To Find Concave Down Intervals By Graphing Functions

Find the intervals that are concave down in  between the range of .

Explanation:

To find which interval is concave down, find the second derivative of the function.

Now, find which  values in the interval specified make . In this case,  and .

Now to find which interval is concave down choose any value in each of the regions

, and

and plug in those values into  to see which will give a negative answer, meaning concave down, or a positive answer, meaning concave up.

A test value of  gives us a  of . This value falls in the range, meaning that interval is concave down.

### Example Question #7 : How To Find Concave Down Intervals By Graphing Functions

The function  is concave-down for what values of  over the interval ?

It is never concave down.

It is concave down over the entire interval

Explanation:

The derivative of  is

The derivative of this is

This is the second derivative.

A function is concave down if its second derivative is less than 0.

whenever

This is true when:

### Example Question #8 : How To Find Concave Down Intervals By Graphing Functions

Given the equation of a graph is , find the intervals that this graph is concave down on.

This graph is always concave up.

Explanation:

To find the concavity of a graph, the double derivative of the graph equation has to be taken. To take the derivative of this equation, we must use the power rule,

.

We also must remember that the derivative of an constant is 0.

After taking the first derivative of the equation using the power rule, we obtain

The double derivative of the equation we are given comes out to

.

Setting the equation equal to zero, we find that .  This point is our inflection point, where the graph changes concavity.  In order to find what concavity it is changing from and to, you plug in numbers on either side of the inflection point. if the result is negative, the graph is concave down and if it is positive the graph is concave up.  Plugging in 2 and 3 into the second derivative equation, we find that the graph is concave up from  and concave down from .

### Example Question #9 : How To Find Concave Down Intervals By Graphing Functions

Determine the intervals on which the following function is concave down:

Explanation:

To find the invervals where a function is concave down, you must find the intervals on which the second derivative of the function is negative. To find the intervals, first find the points at which the second derivative is equal to zero. The first derivative of the function is equal to

.

The second derivative of the function is equal to

.

Both derivatives were found using the power rule

Solving for x, . The intervals, therefore, that we analyze are  and .

On the first interval, the second derivative is negative, which means the function is concave down. On the second interval, the second derivative is positive, which means the function is concave up. (Plug in values on the intervals into the second derivative and see if they are positive or negative.)

Thus, the first interval  is the answer.

### Example Question #10 : How To Find Concave Down Intervals By Graphing Functions

How many infelction points does the function

have on the interval  ?

One

Two

Zero

Three

Three

Explanation:

Points of inflection occur where there second derivative of a function are equal to zero. Taking the first and second derivative of the function, we find:

To find the points of inflection, we find the values of x that satisfy the condition

.

Which occurs at

Within the defined interval [-5, 5], there are three values: .  These points are represented on the figure below as red dots.

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