How can I center a set of equations and make a line under one of them?

Question

This is something I need to achieve, I have a problem with the last equation. How can I underline this the same way as above? I've tried writing it as one fraction, but the other fraction in numerator gets smaller and the equal sign isn't in line with other signs. Could you please advise?

begin{eqnarray*}
\frac{2}{3}=0,66666\ 66667\ (-) \\
\frac{2}{3\cdot 3 \cdot 9}=0,02469\ 13580\ (+) \\
\frac{2}{3\cdot 5 \cdot 9^{2}}=0,00164\ 60905\ (+) \\
\frac{2}{3\cdot 7 \cdot 9^{3}}=0,00013\ 06421\ (+) \\
\frac{2}{3\cdot 9 \cdot 9^{4}}=0,00001\ 12901\ (-) \\
\frac{2}{3\cdot 11 \cdot 9^{5}}=0,00000\ 10264\ (-) \\
\frac{2}{3\cdot 13 \cdot 9^{6}}=0,00000\ 00965\ (-) \\
\frac{2}{3\cdot 15 \cdot 9^{7}}=0,00000\ 00093\ (-) \\
\end{eqnarray*}

This is the code, but don't know what about last part...

please provide what you do so far. Retyping equations from your image is not really fun for us ... — Zarko, Nov 28 '15 at 16:12
You could just draw a horizontal line using \rule or \hline (if in a tabular), — John Kormylo, Nov 28 '15 at 16:15
It looks like you've got two separate accounts, which means you cannot edit your original post or leave comments. The Stack Exchange staff can merge them together for you. — Werner, Nov 28 '15 at 16:34

score 3 · Answer 1 · answered Nov 28 '15 at 16:30

I would construct this in a regular array, provided that there is no requirement for the construction to break across the page boundary. Here is such an implementation:

\documentclass{article}

\usepackage{array}

\begin{document}

\[
  \renewcommand{\arraystretch}{2.1}% http://tex.stackexchange.com/q/31672/5764
  \begin{array}{ >{\displaystyle}r @{} >{{}}l c}
                       \frac{2}{3} &= 0.66666\,66667 & (-) \\
       \frac{2}{3 \cdot 3 \cdot 9} &= 0.02469\,13580 & (+) \\
     \frac{2}{3 \cdot 5 \cdot 9^2} &= 0.00164\,60905 & (+) \\
     \frac{2}{3 \cdot 7 \cdot 9^3} &= 0.00013\,06421 & (+) \\
     \frac{2}{3 \cdot 9 \cdot 9^4} &= 0.00001\,12901 & (-) \\
    \frac{2}{3 \cdot 11 \cdot 9^5} &= 0.00000\,10264 & (-) \\
    \frac{2}{3 \cdot 13 \cdot 9^6} &= 0.00000\,00965 & (-) \\
    \frac{2}{3 \cdot 15 \cdot 9^7} &= 0.00000\,00093 & (-) \\[0.7\normalbaselineskip]
    \hline
                                   &= 0.69314\,71805
  \end{array}
\]

\end{document}

score 3 · Answer 2 · 2015-11-29T11:14:38.903

use the align* environment from amsmath and a simple \cline just like in a tabular

\documentclass{article}
\usepackage{amsmath}
\begin{document}

    \begin{align*}
    \frac{2}{3}                    &= 0.66666\,66667  ~(-) \\
    \frac{2}{3 \cdot 3 \cdot 9}    &= 0.02469\,13580  ~(+) \\
    \frac{2}{3 \cdot 5 \cdot 9^2}  &= 0.00164\,60905  ~(+) \\
    \frac{2}{3 \cdot 7 \cdot 9^3}  &= 0.00013\,06421  ~(+) \\
    \frac{2}{3 \cdot 9 \cdot 9^4}  &= 0.00001\,12901  ~(-) \\
    \frac{2}{3 \cdot 11 \cdot 9^5} &= 0.00000\,10264  ~(-)\\
    \frac{2}{3 \cdot 13 \cdot 9^6} &= 0.00000\,00965  ~(-)\\
    \frac{2}{3 \cdot 15 \cdot 9^7} &= 0.00000\,00093  ~(-)\\\cline{1-2}
                                   &= 0.69314\,71805
    \end{align*}

\end{document}

Zarko · Answer 3 · 2015-11-28T22:14:47.467

As possible starting point:

\documentclass{article}
    \usepackage{array,siunitx}
    \usepackage{mathtools}

    \begin{document}
    \begin{center}
\begin{tabular}{>{$}c<{$}}
    \begin{aligned}
\frac{2}{3}                 & = \num{0,6666666667}\ (-)   \\
\frac{2}{3\cdot 3\cdot 9}   & = \num{0,0246913580}\ (+)   \\
\frac{2}{3\cdot 3\cdot 9^2} & = \num{0,0016460905}\ (+)   \\
\frac{2}{3\cdot 3\cdot 9^3} & = \num{0,0001306421}\ (+)   \\
\frac{2}{3\cdot 3\cdot 9^4} & = \num{0,0000112901}\ (-)   \\
\frac{2}{3\cdot 3\cdot 9^5} & = \num{0,0000010264}\ (-)   \\
\frac{2}{3\cdot 3\cdot 9^6} & = \num{0,0000000965}\ (-)   \\
\frac{2}{3\cdot 3\cdot 9^7} & = \num{0,0000000093}\ (-)   \\[1ex]
    \hline
                            & = \num{0,6931471805}
    \end{aligned}
\end{tabular}
    \end{center}
\end{document}

The package array serves to determine >{$}c<{$}, siunitx for macro \num{...} which enable to grouping numbers in 3. For line is used standard \hline from tabular environment.

@eqgreg, only for draw line. As I see from your answer, this can be achieved without tabular environment with use of cline. — Zarko, Nov 28 '15 at 22:19
I was mainly objecting to tabular instead of array. However, if you're using array, you can avoid the inner align, can't you? — egreg, Nov 28 '15 at 22:21

How can I center a set of equations and make a line under one of them?

3 Answers3