Chapter 8: Infinite Sequences and Series
Section 8.5: Taylor Series
Example 8.5.7
Show that R^n+1x for fx=ln1+x goes to zero as n→∞, establishing that f has a Maclaurin series.
Find the terms of that series.
Solution
Mathematical Solution
The absolute value of the Taylor-series remainder for fx=ln1+x is
|R^n+1x|=x1+xn+1n+1
For each fixed x, R^n+1x→0, as suggested by Figure 8.5.7(a) where n is controlled by the slider.
As n→∞, |R^n+1x|→0 but the limit is not uniform in x. As n increases, the interval on which the remainder gets close to zero increases to the right.
n= =
Figure 8.5.7(a) Slider-controlled graph of R^n+1
Maple Solution
The expression for R^n+1
Write R=… Context Panel: Assign Name
R=x1+xn+1n+1→assign
Show that R^n+1→0 as n→∞
Calculus palette: Limit template Add the positivity assumption! Context Panel: Evaluate and Display Inline
limn→∞R assuming x>0 = 0
Obtain the Maclaurin series
Write the given function.
Context Panel: Series≻Formal Power Series Complete the dialog as per Figure 8.5.7(b).
Figure 8.5.7(b) Formal Power Series dialog
ln1+x→formal series∑n=0∞⁡−1n⁢xn+1n+1
Obtain the Maclaurin series from first principles
Write fx=… Context Panel: Assign Function
fx=ln1+x→assign as functionf
Expression palette: Summation template Calculus palette: nth-derivative template
Context Panel: 2-D Math≻Convert To≻Inert Form
Context Panel: Evaluate and Display Inline
Context Panel: Simplify≻Assuming Integer
∑n=0∞fn0n!xn = ∑n=0∞⁡0n=0−1n−1⁢n−1!otherwise⁢xnn!→assuming integer∑n=0∞⁡0n≤0−−xnn0<n
Although the expressions for the solutions given by the Series≻Formal Power Series option in the Context Panel and by first principles differ, the terms of both series are the same.
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