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## AP®︎ Calculus AB (2017 edition)

### Course: AP®︎ Calculus AB (2017 edition) > Unit 6

Lesson 7: Justifying properties of functions using the second derivative- Inflection points from graphs of function & derivatives
- Justification using second derivative: maximum point
- Justification using second derivative: inflection point
- Justification using second derivative
- Justification using second derivative
- Worked example: Inflection points from first derivative
- Worked example: Inflection points from second derivative
- Inflection points from graphs of first & second derivatives
- Second derivative test
- Second derivative test

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# Justification using second derivative

"Calculus-based reasoning" with the second derivative of a function can be used to justify claims about the concavity of the original function and about its inflection points.

We've learned that the first derivative ${f}^{\prime}$ gives us information about where the original function $f$ increases or decreases, and about where $f$ has extremum points.

The second derivative ${f}^{\u2033}$ gives us information about the $f$ and about where $f$ has inflection points.

*concavity*of the original function## Let's review what concavity is all about.

A function is $\cup $ .

**concave up**when its slope is increasing. Graphically, a graph that's concave up has a cup shape,Similarly, a function is $\cap $ .

**concave down**when its slope is decreasing. Graphically, a graph that's concave down has a cap shape,A

**point of inflection**is where a function changes concavity.## How ${f}^{\u2033}$ informs us about the concavity of $f$

When the second derivative ${f}^{\u2033}$ is positive, that means the first derivative ${f}^{\prime}$ is increasing, which means that $f$ is concave up. Similarly, a negative ${f}^{\u2033}$ means ${f}^{\prime}$ is decreasing and $f$ is concave down.

positive | increasing | concave up |

negative | decreasing | concave down |

crossing | extremum point (changes direction) | inflection point (changes concavity) |

Here’s a graphical example:

Notice how $f$ is ${\text{concave down}}$ to the left of $x=c$ and ${\text{concave up}}$ to the right of $x=c$ .

### Common mistake: Confusing the relationship between $f$ , ${f}^{\prime}$ , and ${f}^{\u2033}$

Remember that for $f$ to be concave up, ${f}^{\prime}$ needs to be increasing and ${f}^{\u2033}$ needs to be positive. Other behaviors of $f$ , ${f}^{\prime}$ , and ${f}^{\u2033}$ aren't necessarily related.

For example, in Problem 1 above, ${f}^{\u2033}$ is concave up over the interval $[-8,-2]$ but it doesn't mean $f$ is concave up on that interval.

*Want more practice? Try this exercise.*

### Common mistake: Misinterpreting the graphical information presented

Imagine a student solving Problem 2 above, thinking that the graph is of the $h$ . In that case, $h$ would have an inflection point at $A$ and $B$ , because these are the points where ${h}^{\prime}$ changes its direction. This student would be wrong, because this is the graph of the $D$ .

*first*derivative of*second*derivative and the correct answer is*Remember to always make sure you understand the information given. Are we given the graph of the function*$f$ , the first derivative ${f}^{\prime}$ , or the second derivative ${f}^{\u2033}$ ?

## Using the second derivative to determine whether an extremum point is a min or a max

Imagine we are given that a function $f$ has an extremum point at $x=1$ , and that it's concave up over the interval $[0,2]$ . Can we tell, based on this information, whether that extremum point is a minimum or a maximum?

The answer is YES. Recall that a function that's concave up has a cup $\cup $ shape. In that shape, a curve can only have a minimum point.

Similarly, if a function is concave down when it has an extremum, that extremum must be a maximum point.

*Want more practice? Try this exercise.*

## Want to join the conversation?

- If the first derivative of a function at c is zero, does that mean that the second derivative at c is also zero?(3 votes)
- Not necessarily. Take x^2. First derivative at 0 is 2*0, which is 0, but its second derivative is just a constant 2, so at x=0 the constant equation 2 is 2 everywhere.

Another way to look at it is the first derivative tells if the slope is 0, and the second derivative will tell if the original function is at an inflection point. If the slope of a function is 0, does that necessarily mean that it is also an inflection point? Any relative extrema disprove this. Let me know if that didn't make sense.(5 votes)

- The graph under the title "How f′′ informs us about the concavity of f" is wrong.

Slope of f at point c should be zero, but it's clearly not the case in the graph.

Correct graph should look something like y=x^3, with appropriate shifts.(0 votes)- No, it really isn't wrong.

The slope at point c does NOT need to be zero. It's the*second*derivative (the slope of the slope as it were) that is zero at an inflection point (a change of concavity).

It is true that y = x³ has an inflection point at x = 0, and that the slope at x = 0 is also 0, but this is just coincidence. It's that fact that f''(0) = 6x = 0 that indicates a change in concavity.

Consider the sine curve. To the left of x = 0 it is clearly "concave up" - it's a U shape. To the right it's clearly concave down a sort of ∩ shape. the change occurs at x = 0, even though the slope at x = 0 is obviously greater than zero. (cos(0) = 1). However the second derivative (-sin(0))*is*zero at that point.(6 votes)

- I have a quick clarification. Just knowing that f′′(x) (the second derivative of f(x)) is less than zero, does that tell me right there without knowing anything else that there is a maximum point. That's what I was confused at, because that's what I thought Sal was saying, but that can't be right, because there are lot's of times that second derivatives of functions are negative, but it's not a relative maximum.

Isn't this what's right: If f′′(x) < 0 && f′(x) === 0 then you have a relative maximum? Or if you have f′′(x) > 0 && f′(x) === 0 you have a relative minimum?

Thanks in advance. :)

- ncochran2(2 votes)- There is a relative maximum if:

1. f''(x) < 0 and f'(x) = 0 if the function is twice-differentiable at x

Example: -x^2, x = 0

2. f'(x) changes from positive to negative around x, and f'(x) = 0 or undefined, for f once-differentiable around x (really the same thing, just if you have a cusp)

Example: -|x|, x = 0

3. f has a point discontinuity at x, but the point is a relative maximum

Example: -x^2 if x ≠ 0, 1 if x = 0

4. Miscellaneous things: jump discontinuities, everywhere continuous but nowhere differentiable functions, almost continuous functions, nowhere continuous functions, etc. Don't worry about these.

Example: Weierstrass function

So yeah. Don't forget about if f' is undefined.(1 vote)

- let g be a twice differentiable function, let g(4)=-2, g'(4)=0, g''(4)=6. What occurs in the graph at the point (4,-2)(1 vote)
- Since g’(4) = 0, the point (4,-2) is a critical point. Since g’’(4) is positive, this critical point is a
**relative minimum**.

Have a blessed, wonderful day!(2 votes)

- Is it reasonable to not use double derivation, but rather use a graphical drawing to make a justification of why it is a maximum or a minimum?(1 vote)
- Yes it is not an issue if we use a graph to show the maximum or minimum of a function. However, for unbounded functions this method needs to be used carefully.(2 votes)

- what does the minima/maxima of the second derivative represent?(1 vote)