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Ch. 10 - Infinite Sequences and Series
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077당신이 사용하는 게 아니라요?교과서 변경
10장, 문제 10.9.33

Find the first four nonzero terms in the Maclaurin series for the functions in Exercises 31–38.
(tan⁻¹x)²

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Recall that the Maclaurin series is the Taylor series expansion of a function about \(x=0\). We want to find the first four nonzero terms of the Maclaurin series for the function \(f(x) = (\tan^{-1} x)^2\).
Start by writing the Maclaurin series for \(\tan^{-1} x\). The known expansion is: \[ \tan^{-1} x = \sum_{n=0}^{\infty} (-1)^n \frac{x^{2n+1}}{2n+1} = x - \frac{x^3}{3} + \frac{x^5}{5} - \frac{x^7}{7} + \cdots \]
Since \(f(x) = (\tan^{-1} x)^2\), square the series for \(\tan^{-1} x\) up to the terms that will produce the first four nonzero terms in \(f(x)\). This means considering terms up to at least \(x^7\) in \(\tan^{-1} x\) to get terms up to \(x^8\) in \(f(x)\).
Perform the multiplication: \[ \left(x - \frac{x^3}{3} + \frac{x^5}{5} - \frac{x^7}{7} + \cdots \right)^2 \] Multiply out the terms carefully, combining like powers of \(x\), and keep track of the coefficients for each power.
Collect the terms by powers of \(x\) and identify the first four nonzero terms in the resulting series. These will be the first four nonzero terms of the Maclaurin series for \((\tan^{-1} x)^2\).

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주요 개념

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Maclaurin Series

A Maclaurin series is a Taylor series expansion of a function about zero. It expresses the function as an infinite sum of terms involving derivatives evaluated at zero, allowing approximation near x = 0. Understanding how to find and use these series is essential for expanding functions like (tan⁻¹x)².
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Series Expansion of Inverse Trigonometric Functions

The inverse tangent function, tan⁻¹x, has a known Maclaurin series expansion involving odd powers of x with alternating signs. Recognizing and using this series is crucial for finding the expansion of (tan⁻¹x)² by substituting and manipulating the series terms.
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Derivatives of Other Inverse Trigonometric Functions

Multiplication and Collection of Series Terms

To find the series for (tan⁻¹x)², one must square the Maclaurin series of tan⁻¹x and combine like powers of x. This involves multiplying series term-by-term and carefully collecting coefficients of each power to identify the first four nonzero terms.
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