11

I am trying to subtract one equation from another but I can't get it to display the way that I want. This is what I have right now.

\begin{align*}
&y&=2x+5\\   
-&&\\    
&y&=3x+10\\\hline   
&0&=-x-5    
\end{align*}
Werner
  • 603,163
russjohnson09
  • 279

5 Answers5

18

This does not look very good but seems close to what your code is attempting to do, where I use \cline{} to draw the horizontal line, and \phantom{y=} to push the minus sign to the left

enter image description here

Recomended Solution:

However, I would suggest the use of \intertext or \shortintertext from the mathtools package, and refer to the equations:

enter image description here

Notes:

  • Requires two runs: first run will display a (??) in the cross reference.
  • \shortintertext yields better spacing but requires an additional package. Alternatively, you can use \intertext which is available in amsmath.
  • The mathtools package already includes amsmath.

Code:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
    y&=2x+5\\   
    -\phantom{y=}&\\    
    y&=3x+10\\
   \cline{1-2}
    0&=-x-5    
\end{align*}
\end{document}

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{align}
    y&=2x+5 \label{eqn:one}\\
    y&=3x+10 \label{eqn:two}\\
  \shortintertext{Subtracting \eqref{eqn:two} from \eqref{eqn:one} yields}
    0&=-x-5  \notag
\end{align}
\end{document}
Community
  • 1
Peter Grill
  • 223,288
12

My two-cents worth, using more of a textual representation and an array:

enter image description here

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\[
  \begin{array}{l@{\quad}cr@{}l}
    && y & {}= 2x+5 \\
    \text{minus} && y & {}= 3x+10 \\ \cline{2-4}
    && 0 & {}= -x-5
  \end{array}
\]
\end{document}​

The additional (second) column is to allow for the \cline to have a little padding around the equations. Modify \quad to \hspace{<len>} to have a gap of length <len> between "minus" and the equation.

Werner
  • 603,163
7

Here is my solution: enter image description here

\documentclass[]{article}

\usepackage{amsmath}
\begin{document}
\begin{alignat*}{4}
    &y&=&&2x&&+&5\\   
    -\hspace*{.5em}&y&=&&3x&&+&10\\[-2.5ex]\cline{2-8}\\[-4.8ex]
    &0&=&&-x&&-&5    
\end{alignat*}

\end{document}

I use the alignat enviroment from amsmath

someonr
  • 8,531
  • 1
    Good one. Your spacing around the = on the second line is wrong. try removing the first & on each line. Hmmmm, on second thought, the location of the minus sign means "negative y", not minus the entire equation... – Peter Grill Feb 01 '12 at 20:03
  • I already noted the problem. and posted the corrected version. thx ;) – someonr Feb 01 '12 at 20:05
  • 1
    Actually you should just use a single & to get the proper math spacing. Still there is a problem with the meaning of "minus y", which is not the OP's intent. – Peter Grill Feb 01 '12 at 20:07
  • Yeah ok. I fixed it... But I'm not sure what russjohnson09 would like to see – someonr Feb 01 '12 at 20:20
1

Stealing the idea with \frac from this answer but replacing the matrix with aligned. Is it a cute idea? I hope so!

\documentclass[preview,border=12pt,varwidth]{standalone}

\usepackage{amsmath}
\begin{document}
\abovedisplayskip=0pt\relax
\[
    \frac
    {
        \!\begin{aligned} 
                y &=    2x+5\\ 
                y   &=  3x+10 
        \end{aligned}
    }
    {0=-x-5}
    \ -
\]
\end{document}

enter image description here

Community
  • 1
kiss my armpit
  • 36,086
0

I write it like this:

\[\frac{\begin{matrix} y=2x+5\\ y=3x+10 \end{matrix}}{0=-x-5}\ -\]
kalakay
  • 2,469