Find the Error: Solving Homework Problems

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SUMMARY

The discussion focuses on identifying the error in a mathematical solution related to a homework problem involving derivatives of a series. The user initially arrives at an incorrect answer of x/2 instead of the expected 0. The key insight provided is that the term y' (x) contains only odd powers of x, which affects the derivative calculation. Specifically, the term n=1 does not vanish, leading to the additional x/2 that needs to be canceled out.

PREREQUISITES
  • Understanding of calculus, specifically derivatives of series.
  • Familiarity with power series and their convergence.
  • Knowledge of Taylor series expansion and its terms.
  • Ability to manipulate and simplify algebraic expressions involving series.
NEXT STEPS
  • Review the properties of power series and their derivatives.
  • Study Taylor series and how to derive them for various functions.
  • Learn about odd and even functions in relation to series expansion.
  • Practice solving similar calculus problems to reinforce understanding of series derivatives.
USEFUL FOR

Students studying calculus, particularly those tackling series and derivatives, as well as educators looking for examples of common mistakes in mathematical problem-solving.

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Harmony said:
May I know which part of my solution went wrong? In the end the answer gives x/2 instead of 0.

Since [tex]y' (x)[/tex] contains only odd powers of x, the n=1 term does not vanish when you take the derivative of the series. Therefore,

[tex]y'' (x)= \sum_{n=1}^{\infty} \frac{(-1)^n (2n)(2n-1) x^{2n-2}}{4^n (n!)^2}[/tex]

The extra [tex]x/2[/tex] in your answer will be canceled by the [tex]n=1[/tex] term.
 

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