Proving the Second Derivative Using L'Hopital's Rule

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SUMMARY

The limit expression lim(h-->0) [f(x+h)-2f(x) + f(x-h)]/h^2 equals f''(x) when the second derivative exists. The application of L'Hôpital's Rule simplifies the expression to [f(x+h)-f(x-h)]/2h, which represents the average of the left and right limits for the derivative of f(x). It is crucial to assume that f is differentiable in an entire neighborhood of x to validly apply L'Hôpital's Rule in this context.

PREREQUISITES
  • Understanding of limits and continuity in calculus
  • Familiarity with L'Hôpital's Rule
  • Knowledge of derivatives and second derivatives
  • Concept of differentiability in a neighborhood
NEXT STEPS
  • Study the application of L'Hôpital's Rule in various limit problems
  • Learn about the conditions for differentiability and its implications
  • Explore the relationship between derivatives and Taylor series expansions
  • Investigate the proofs of the second derivative test in calculus
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Students studying calculus, particularly those focusing on limits, derivatives, and the application of L'Hôpital's Rule in proving derivative properties.

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Homework Statement



Show that lim(h-->0) [f(x+h)-2f(x) + f(x-h)]/h^2
is equal to f''(x) for any given value of x where the second derivative exists.


I'm supposed to use l'hospital's rule for this problem. I did and got
[f(x+h)-f(x-h)]/2h

Now I am stuck. I thought about adding and subtracting say f(x) but don't see how that solves to f''(x)
 
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You're almost there. You can rewrite that as the average of the left and right limits for the derivative of f(x), which will be the same if f is differentiable at x.
 
f(x+h)-f(x-h)

is not the derivative of

f(x+h)-2f(x) + f(x-h)


p.s.: for the record, you have to assume f is differentiable in an entire neighborhood of x in order to invoke L'Hôpital's rule.
 
Last edited:

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