Math & Science ⇒ How to put a horizontal line in eqnarray

[twisterrob]

In high school we used to solve a system of equation like this:

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x + y = 10
x - y = 5
----------------
x + y = 10
    x = 5 + y
----------------
(5 + y) + y = 10
     5 + 2y = 10
         2y = 5
          y = 2.5
          x = 5 + 2.5 = 7.5

Now I would like to write an equation array like this:

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\begin{eqnarray*}
x + y &=& 10\\
x - y &=& 5\\
\hline % gives compliation error, that \hline command does not exists
x + y &=& 10\\
x &=& 5 + y\\
\hline % gives compliation error, that \hline command does not exists
(5 + y) + y &=& 10\\
5 + 2y &=& 10\\
2y &=& 5\\
y &=& 2.5\\
x &=& 5 + 2.5 = 7.5
\end{eqnarray*}

I would like to know how to do it! What to write instead of \hline? (I know how to do with tables/tabulars (where \hline simply just works), I want to do it without tabular or array env) with eqnarray like enviroment (maybe align?)

[localghost]

At the moment I can only offer an intermediate solution.

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\begin{align*}
  x + y &= 10 \\
  x - y &= 5 \\
  ------&-------- \\
  x + y &= 10 \\
  x &= 5 + y \\
  ------&-------- \\
  (5 + y) + y &= 10 \\
  5 + 2y &= 10 \\
  2y &= 5 \\
  y &= 2.5 \\
  x &= 5 + 2.5 = 7.5
\end{align*}

The horizontal lines are dashed.


Best regards
Thorsten¹

[James]

You can use \cline:

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\begin{align*}
  x + y &= 10 \\
  x - y &= 5 \\
  \cline{1-2}
  x + y &= 10 \\
  x &= 5 + y \\
  \cline{1-2}
  (5 + y) + y &= 10 \\
  5 + 2y &= 10 \\
  2y &= 5 \\
  y &= 2.5 \\
  x &= 5 + 2.5 = 7.5
\end{align*}

[Stefan Kottwitz]

Hi,

don't use eqnarray, see eqnarray vs. align for a short explanation.
You could see the problem already in your example, just look at the spacing around the equal signs of the last line.

Stefan

[Lauren]

I am using the \cline workaround suggested above, and have found that a large amount of vertical whitespace is inserted before the horizontal line, so I do this:

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    \begin{align*}
      x + y &= 10 \\
      x - y &= 5 \\[-11pt]
      \cline{1-2}
      x + y &= 10 \\
      x &= 5 + y \\[-11pt]
      \cline{1-2}
      (5 + y) + y &= 10 \\
      5 + 2y &= 10 \\
      2y &= 5 \\
      y &= 2.5 \\
      x &= 5 + 2.5 = 7.5
    \end{align*}

This seems less than optimal, although it works. Does anyone have a better suggestion?