Definition of second derivative as a limit

Question

I found a statement that the second derivative can be defined as:

$$\lim_{x \to a} \frac{f '(x)-f '(a)}{x-a}$$.

Does this definion follow from the definition of the first derivative as:

$$f ' (x) = \lim_{h \to 0} \frac{f(x+h)-f(x)}{h}$$

If so, how? If not, where does it come from?

Edit: Mistake corrected, sorry.

Your first expression makes no sense at all: first, there is $;n\to a;$ (perhaps you meant $;x\to a;$?) , and even then that's the definition of the first derivative at $;a;$ ... — DonAntonio, Feb 25 '14 at 17:08
Perhap you meant the "Second definition" rather than "second derivative"? — SomeStrangeUser, Feb 25 '14 at 17:09

BigM · Answer 1 · 2020-02-12T15:14:25.870

15

Assuming that $f$ is $C^2$ then $\displaystyle\lim_{h\rightarrow 0} \frac{f(x+h)-2f(x)+f(x-h)}{h^2}=^{\text{L'Hospital's}}\displaystyle\lim_{h\rightarrow 0} \frac{f'(x+h)-f'(x-h)}{2h}$

$=^{\text{L'Hospital's}}\displaystyle\lim_{h\rightarrow 0} \frac{f''(x+h)+f''(x-h)}{2}=f''(x) $

edited Feb 12 '20 at 15:14

answered Oct 29 '19 at 18:32

BigM

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score 11 · Answer 2 · answered Feb 25 '14 at 17:27

11

Yes, since

$$f'(x) = \lim_{h\rightarrow 0} \frac{f(x+h)-f(x)}{h},$$

and the second derivative is the derivative of the derivative, we get

$$f''(x) = \lim_{h\rightarrow 0} \frac{f'(x+h)-f'(x)}{h}.$$

There are also difference quotients for the second derivative defined immediately in terms of $f$. The most commonly seen is

$$f''(x) = \lim_{h\rightarrow 0} \frac{f(x+h)-2f(x)+f(x-h)}{h^2}.$$

This is commonly derived using Taylor expansions.

answered Feb 25 '14 at 17:27

Mark McClure

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Silly question but, How are the two versions of of $f''(x)$ the same – homosapien Nov 04 '21 at 21:34

score 3 · Answer 3 · edited Jul 24 '20 at 11:30

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The second derivative is defined applying the definition of derivative to the first derivative, i.e.: $$ f''(x)=\lim_{h\to0}\frac{f'(x+h)-f'(x)}{h}, $$ where: $$ f'(y)=\lim_{h\to0}\frac{f(y+h)-f(y)}{h}. $$ I do not think the first expression you wrote makes any sense. What is $n$? If you meant $x$, that is the definition of the first derivative.

edited Jul 24 '20 at 11:30

BigM

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answered Feb 25 '14 at 17:07

7raiden7

1,794

Sasha Patotski · Accepted Answer · 2014-02-25T17:34:52.357

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The first formula does not define you the second derivative of $f$ at $a$, only the first derivative. These two definitions are equivalent. If you put $h=a-x$, you get one definition from the other.

And also, in the first formula under the limit sign you should have $a\to x$

edited Feb 25 '14 at 17:34

answered Feb 25 '14 at 17:06

Sasha Patotski

7,161

I did this, but I am getting $\frac{f(2x-a)-f(x)}{x-a}$. How is this equivalent to the desired fraction in the limit? – kiwifruit Feb 25 '14 at 17:17
@kiwifruit There should be $h=a-x$. I have corrected it. – Sasha Patotski Feb 25 '14 at 17:49
Ok, but then I get $\frac{f(a)-f(x)}{a-x}$. Is that equivalent to reversing both subtractions? – kiwifruit Feb 25 '14 at 18:12

score 1 · Answer 5 · answered Feb 25 '14 at 17:06

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This is not a definition for the second derivative. This is an alternative definition for the first derivative.

answered Feb 25 '14 at 17:06

lemon

3,548

Definition of second derivative as a limit

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