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Ch. 11 - Power Series
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243Non è quello che usi tu?Cambia libro di testo
Capitolo 11, Problema 11.2.9

Radius and interval of convergence Determine the radius and interval of convergence of the following power series.


∑ₖ₌₀∞ (2x)ᵏ

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1
Identify the given power series: \(\sum_{k=0}^{\infty} (2x)^k\). This is a geometric series with the general term \(a_k = (2x)^k\).
Recall that a geometric series \(\sum_{k=0}^{\infty} r^k\) converges if and only if \(|r| < 1\). Here, the ratio \(r\) is \$2x$.
Set up the inequality for convergence: \(|2x| < 1\). This inequality will help us find the radius of convergence.
Solve the inequality \(|2x| < 1\) to find the interval for \(x\): dividing both sides by 2 gives \(|x| < \frac{1}{2}\). This means the radius of convergence \(R\) is \(\frac{1}{2}\).
Determine the interval of convergence by considering the endpoints \(x = -\frac{1}{2}\) and \(x = \frac{1}{2}\). Test these values in the original series to check if the series converges or diverges at the endpoints.

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Power Series

A power series is an infinite sum of terms in the form a_k(x - c)^k, where a_k are coefficients and c is the center. Understanding power series helps analyze functions as infinite polynomials and study their convergence behavior around the center point.
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Intro to Power Series

Radius of Convergence

The radius of convergence is the distance from the center within which a power series converges absolutely. It can be found using tests like the Ratio Test, and it defines the interval where the series represents a valid function.
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Radius of Convergence

Interval of Convergence

The interval of convergence is the set of x-values for which the power series converges. It includes all points within the radius of convergence and requires checking endpoints separately to determine if the series converges there.
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Interval of Convergence
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Suppose you know the Maclaurin series for f and that it converges to f(x) for |x|<1. How do you find the Maclaurin series for f(x²) and where does it converge?

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{Use of Tech} A different kind of approximation When approximating a function f using a Taylor polynomial, we use information about f and its derivative at one point. An alternative approach (called interpolation) uses information about f at several different points. Suppose we wish to approximate f(x)=sin x on the interval [0, π].


a. Write the (quadratic) Taylor polynomial p₂ for f centered at π/2.


b. Now consider a quadratic interpolating polynomial q(x) = ax² + bx + c. The coefficients a, b, and c are chosen such that the following conditions are satisfied:

q(0) = f(0), q(π/2) = f(π/2), and q(π) = f(π)


Show that q(x) = −(4/π²)x² + (4/π)x.


c. Graph f, p₂, and q on [0, π].


d. Find the error in approximating f(x) = sin x at the points π/4, π/2, 3π/4, and π using p₂ and q.


e. Which function, p₂ or q, is a better approximation to f on [0, π]? Explain.

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Probability: tossing for a head The expected (average) number of tosses of a fair coin required to obtain the first head is ∑ₖ₌₁∞ k(1/2)ᵏ. Evaluate this series and determine the expected number of tosses. (Hint: Differentiate a geometric series.) 

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{Use of Tech} Estimating errors Use the remainder to find a bound on the error in approximating the following quantities with the nth-order Taylor polynomial centered at 0. Estimates are not unique.


sin 0.3, n = 4

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Any method


a. Use any analytical method to find the first four nonzero terms of the Taylor series centered at 0 for the following functions. You do not need to use the definition of the Taylor series coefficients.


b. Determine the radius of convergence of the series.


f(x) = (1 + x²)⁻²/³

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Manipulating Taylor series Use the Taylor series in Table 11.5 to find the first four nonzero terms of the Taylor series for the following functions centered at 0.


ln (1 + x²)

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