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29.03.2015
 #5
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29.03.2015
 #3
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$$\lim \limits_{n\rightarrow +\infty} {\sqrt{(n^2-n)}-n } =\ ?$$

Wir formen um:

$$=\lim \limits_{n\rightarrow +\infty} {\sqrt{n(n-1)}-n }\\
=\lim \limits_{n\rightarrow +\infty} {\sqrt{\frac{n}{n}\cdot n(n-1)}-n }\\
=\lim \limits_{n\rightarrow +\infty} {\sqrt{\frac{1}{n}\cdot n^2(n-1)}-n }\\
=\lim \limits_{n\rightarrow +\infty} { n \sqrt{\frac{n-1}{n}}-n }\\
=\lim \limits_{n\rightarrow +\infty} { n \sqrt{1-\frac{1}{n}}-n }\\
=\lim \limits_{n\rightarrow +\infty} { n \textcolor[rgb]{1,0,0}{\left(1-\frac{1}{n}\right)^{\frac{1}{2}} } -n }\\$$

Die Taylorreihen-Entwicklung von \sqrt{x} mit Entwicklungspunkt 1

\begin{align} \sqrt{1+x} = \sum_{n=0}^\infty \binom{1/2}{n}\,x^n &= \sum_{n=0}^\infty \binom{2n}{n}\, \frac{ (-1)^n }{ (1-2n)\, 4^n }\,x^n\\ &= 1 + \frac{1}{2}x - \frac{1}{8}x^2 + \frac{1}{16} x^3 - \frac{5}{128} x^4 \pm \dots \end{align}

Wir setzen für  $$\small{\text{$x=-\frac{1}{n}$}}$$

$$\small{\text{
$
\left(1-\frac{1}{n}\right)^{\frac{1}{2}}
=
1
+ \dfrac{1}{2}\cdot \left( -\dfrac{1}{n} \right)
- \dfrac{1}{8}\cdot \left( -\dfrac{1}{n} \right)^2
+ \dfrac{1}{16}\cdot \left( -\dfrac{1}{n} \right)^3
- \dfrac{5}{128}\cdot \left( -\dfrac{1}{n} \right)^4
\pm \cdots
$
}}\\\\
\small{\text{
$
=
1
- \dfrac{1}{2}\cdot \left( \dfrac{1}{n} \right)
- \dfrac{1}{8}\cdot \left( \dfrac{1}{n} \right)^2
- \dfrac{1}{16}\cdot \left( \dfrac{1}{n} \right)^3
- \dfrac{5}{128}\cdot \left( \dfrac{1}{n} \right)^4
\pm \cdots
$
}}\\\\
\small{\text{$
{ n \textcolor[rgb]{1,0,0}{\left(1-\frac{1}{n}\right)^{\frac{1}{2}} } -n }
=
n\cdot \left[
1
- \dfrac{1}{2}\cdot \left( \dfrac{1}{n} \right)
- \dfrac{1}{8}\cdot \left( \dfrac{1}{n} \right)^2
- \dfrac{1}{16}\cdot \left( \dfrac{1}{n} \right)^3
- \dfrac{5}{128}\cdot \left( \dfrac{1}{n} \right)^4
\pm \cdots \right] -n
$}}\\\\
\small{\text{$
{ n \textcolor[rgb]{1,0,0}{\left(1-\frac{1}{n}\right)^{\frac{1}{2}} } -n }
=
\not{n}
- \dfrac{1}{2}\cdot \left( \dfrac{n}{n} \right)
- \dfrac{1}{8}\cdot \left( \dfrac{1}{n^1} \right)
- \dfrac{1}{16}\cdot \left( \dfrac{1}{n^2} \right)
- \dfrac{5}{128}\cdot \left( \dfrac{1}{n^3} \right)
\pm \cdots -\not{n}
$}}\\\\
\small{\text{$
{ n \textcolor[rgb]{1,0,0}{\left(1-\frac{1}{n}\right)^{\frac{1}{2}} } -n }
=
- \dfrac{1}{2}\cdot \left( \dfrac{n}{n} \right)
- \dfrac{1}{8}\cdot \left( \dfrac{1}{n^1} \right)
- \dfrac{1}{16}\cdot \left( \dfrac{1}{n^2} \right)
- \dfrac{5}{128}\cdot \left( \dfrac{1}{n^3} \right)
\pm \cdots
$}}\\\\
\small{\text{$
{ n \textcolor[rgb]{1,0,0}{\left(1-\frac{1}{n}\right)^{\frac{1}{2}} } -n }
=
- \dfrac{1}{2}
- \dfrac{1}{8}\cdot \left( \dfrac{1}{n^1} \right)
- \dfrac{1}{16}\cdot \left( \dfrac{1}{n^2} \right)
- \dfrac{5}{128}\cdot \left( \dfrac{1}{n^3} \right)
\pm \cdots
$}}\\\\
\small{\text{
$
\lim \limits_{n\rightarrow +\infty} \left[ - \dfrac{1}{2}
- \dfrac{1}{8}\cdot \left( \dfrac{1}{n^1} \right)
- \dfrac{1}{16}\cdot \left( \dfrac{1}{n^2} \right)
- \dfrac{5}{128}\cdot \left( \dfrac{1}{n^3} \right)
\pm \cdots \right] = -\dfrac{1}{2}
$}}$$

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29.03.2015

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