Solve Derivative Problem: Lim {P(x+3h)+P(x-3h)-2P(x)}/h^2

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Discussion Overview

The discussion revolves around evaluating the limit of the expression lim {P(x+3h)+P(x-3h)-2P(x)}/h^2 as h approaches 0, where P is a polynomial function. Participants explore various methods to solve this limit problem, including Taylor series expansions and L'Hôpital's rule.

Discussion Character

  • Mathematical reasoning, Homework-related

Main Points Raised

  • One participant presents the limit problem and suggests that the answer might be D) 9P''(x), seeking confirmation and explanation.
  • Another participant proposes using Taylor series expansions for P(x+3h) and P(x-3h) to verify the initial guess, noting that higher order terms vanish as h approaches 0.
  • A different approach is introduced by another participant, who suggests applying L'Hôpital's rule to differentiate the numerator and denominator twice, leading to a different expression involving P''(x) evaluated at specific points.

Areas of Agreement / Disagreement

There is no consensus on the correct answer to the limit problem, as participants propose different methods and interpretations without agreeing on a definitive solution.

Contextual Notes

The discussion includes various mathematical techniques that may depend on the assumptions about the polynomial function P and its derivatives. The application of Taylor series and L'Hôpital's rule introduces different perspectives on how to approach the limit.

lhuyvn
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Hi members,

Could anyone help me with the problem following?

If x is a real number and P is a polynomial function, then


lim {P(x+3h)+P(x-3h)-2P(x)}/h^2
h->0


A)0
B)6P'(x)
C)3P''(x)
D)9P''(x)
E) 00

I guess D should be the answer, I need an explanation.

Thank You
 
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The easiest way is to use Taylor-series expansions of the terms P(x+3h), P(x-3h)
to verify your guess.
Then we have, for example:
P(x+3h)=P(x)+P'(x)3h+1/2P''(x)(3h)^2+O(h^3)

O(h^3) is a higher order term, i.e lim h->0 O(h^3)/h^2=0
 
Thank you, arildno

It couldn't be more wonderful solutions.
 
It looks messy, but it's really no different from any other limit problem. What is typically the easiest way to find the limit of 0/0? L'Hopital's rule!

Differentiate top and bottom with respect to h (not x!) twice, using chain rule for top terms. So the first differentiation gives [3P'(x+3h)-3P'(x-3h)]/2h (notice the third term has no h, so drops out). The second round, you get [9P''(x+3h)+9P''(x-3h)]/2. Then setting h=0 gives the desired answer.
 

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