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Ch. 11 - Power Series
Briggs - Calculus: Early Transcendentals 3rd Edition
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11장, 문제 11.3.25c

Taylor series and interval of convergence


c. Determine the interval of convergence of the series.


f(x) = ln (x − 2), a = 3

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Start by writing the Taylor series expansion of the function \(f(x) = \ln(x - 2)\) centered at \(a = 3\). Recall that the Taylor series of \(f(x)\) about \(a\) is given by \(\displaystyle \sum_{n=0}^{\infty} \frac{f^{(n)}(a)}{n!} (x - a)^n\).
Find the derivatives of \(f(x) = \ln(x - 2)\) evaluated at \(x = 3\). Note that \(f(x)\) can be rewritten as \(\ln(u)\) where \(u = x - 2\), so \(f(x) = \ln(u)\) and \(u\) is shifted by 1 at \(x=3\) since \(u = 3 - 2 = 1\).
Express the general term of the Taylor series using the \(n\)-th derivative of \(f\) at \(a=3\). For \(f(x) = \ln(x-2)\), the \(n\)-th derivative has a known pattern involving factorials and powers of \((x-2)\), which you can use to write the general term \(c_n (x-3)^n\).
Determine the radius of convergence \(R\) of the Taylor series by applying the Ratio Test or Root Test to the general term of the series. This involves taking the limit as \(n \to \infty\) of the ratio of consecutive terms or the \(n\)-th root of the absolute value of the \(n\)-th term.
Once you find the radius of convergence \(R\), write the interval of convergence as \((a - R, a + R)\), i.e., \((3 - R, 3 + R)\). Then check the endpoints \(x = 3 - R\) and \(x = 3 + R\) by substituting back into the original series to see if the series converges or diverges at these points, thus determining the full interval of convergence.

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Taylor Series Expansion

A Taylor series represents a function as an infinite sum of terms calculated from the derivatives at a single point a. For f(x) = ln(x - 2) centered at a = 3, the series approximates the function near x = 3 using derivatives evaluated at that point.
추천 영상:
08:42
Taylor Series

Radius and Interval of Convergence

The radius of convergence is the distance from the center a within which the Taylor series converges to the function. The interval of convergence is the actual range of x-values where the series converges, determined by testing endpoints after finding the radius.
추천 영상:
07:36
Radius of Convergence

Domain Restrictions and Singularities

The function ln(x - 2) is defined only for x > 2, so the series cannot converge outside this domain. The singularity at x = 2 limits the radius of convergence, as the series cannot converge beyond the nearest point where the function is undefined.
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가이드 코스
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Finding the Domain and Range of a Graph