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redthumb edited ratio.tex
over 9 years ago
Commit id: 1d90f4276d484fafda719f8d30d8c12b9e12c8b8
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diff --git a/ratio.tex b/ratio.tex
index e69de29..6278894 100644
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\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\title{\LaTeX}
\date{}
\begin{document}
Let:
\begin{align}
S=\sum^{\infty}_{n=1} n\cdot\left(\frac{5}{4}\right)^{(2n\:+\:1)}
\end{align}
If I use the Ratio Test to determine whether S converges, I need to
determine:
\begin{align}
\lim_{n\to\infty}\left|\:a(n\:+\:1)\hspace{1em}\div\hspace{1em}a(n)\:\right|
\end{align}
What is the value of this limit?
\begin{align}
L=\lim_{n\to\infty}\left|\:a(n\:+\:1)\hspace{1em}\div\hspace{1em}a(n)\:\right| \\
L=\lim_{n\to\infty}\left|\:(n\:+\:1)\cdot\left(\frac{5}{4}\right)^{(2(n\:+\:1)\:+\:1)}\hspace{1em}\div\hspace{1em}n\cdot\left(\frac{5}{4}\right)^{(2n\:+\:1)}\:\right| \\
L=\lim_{n\to\infty}\left|\:(n\:+\:1)\cdot\left(\frac{5}{4}\right)^{(2n\:+\:2\:+\:1)}\hspace{1em}\div\hspace{1em}n\cdot\left(\frac{5}{4}\right)^{(2n\:+\:1)}\:\right| \\
L=\lim_{n\to\infty}\left|\:(n\:+\:1)\cdot\left(\frac{5}{4}\right)^{(2n\:+\:3)}\hspace{1em}\div\hspace{1em}n\cdot\left(\frac{5}{4}\right)^{(2n\:+\:1)}\:\right| \\
L=\lim_{n\to\infty}\left|\:(n\:+\:1)\cdot\left(\frac{5}{4}\right)^{(2n\:+\:3)}\hspace{1em}\cdot\hspace{1em}\frac{1}{n\cdot\left(\frac{5}{4}\right)^{(2n\:+\:1)}}\:\right| \\
L=\lim_{n\to\infty}\left|\:(n\:+\:1)\cdot\left(\frac{5}{4}\right)^{(2n\:+\:3)}\cdot\left(\frac{5}{4}\right)^{-(2n\:+\:1)}\hspace{1em}\cdot\hspace{1em}\frac{1}{n}\:\right| \\
L=\lim_{n\to\infty}\left|\:(n\:+\:1)\cdot\left(\frac{5}{4}\right)^{(2n\:+\:3)}\cdot\left(\frac{5}{4}\right)^{(-2n\:-\:1)}\hspace{1em}\cdot\hspace{1em}\frac{1}{n}\:\right| \\
L=\lim_{n\to\infty}\left|\:(n\:+\:1)\cdot\left(\frac{5}{4}\right)^{(2n\:+\:3\:-\:2n\:-\:1)}\hspace{1em}\cdot\hspace{1em}\frac{1}{n}\:\right| \\
L=\lim_{n\to\infty}\left|\:(n\:+\:1)\cdot\left(\frac{5}{4}\right)^{2}\hspace{1em}\cdot\hspace{1em}\frac{1}{n}\:\right| \\
L=\lim_{n\to\infty}\left|\:(n\:+\:1)\cdot\left(\frac{25}{16}\right)\hspace{1em}\cdot\hspace{1em}\frac{1}{n}\:\right| \\
L=\lim_{n\to\infty}\left|\:\frac{25(n\:+\:1)}{16n}\:\right| \\
L=\lim_{n\to\infty}\left|\:\frac{25n\left(\frac{n}{n}\:+\:\frac{1}{n}\right)}{16n}\:\right| \\
L=\lim_{n\to\infty}\left|\:\frac{25\left(1\:+\:\frac{1}{n}\right)}{16}\:\right| \\
L=\left|\:\frac{25\left(1\:+\:\frac{1}{\infty}\right)}{16}\:\right| \\
L=\left|\:\frac{25\left(1\:+\:0\right)}{16}\:\right| \\
L=\left|\:\frac{25\left(1\right)}{16}\:\right| \\
L=\left|\:\frac{25}{16}\:\right| \\
L=\left[\frac{25}{16}\right]
\end{align}
If $L < 1$ : $S$ Converges\\
\vspace{1em}
If $L > 1$ or $\infty$: $S$ Diverges \\
\vspace{1em}
If $L = 1$: S Inconclusive \\
\vspace{1em}
Using the above answer, we know that $S$ \\
\vspace{1em}
$L=\frac{25}{16} > 1$ : $S$ Diverges \\
\end{document}