[texhax] \hline in align environment

[Sebastian Canagaratna]

When discussing elimination of variables in simultaneous equations, it 
is usual to have a horzontal line after the two equations which are 
being added and subtracted. This appears to be difficult to do in 
amsmath align environment. There was a question about this about three 
years ago, but there were no satisfactory answers usign align, as far as 
I can gather. Here is my solution.

\begin{align*}
       4x+  6y -2z &= -4\\
        x -y +2z   &=  9\\[-0.2in] 
\underline{\phantom{4x+6y-2z}}&\underline{\phantom{y}}\underline{\phantom{-4y}}\\
        5x + 5y    &= 5\\
         x + y &= 1
\end{align*}
I have used underline and for alignment, instead of using \phantom{=} it 
is necessary to use \phantom{y} otherwise the lines don't algin 
horizontally. Similarly the added y in -4y.

It is because of this alignment problem that the 2nd equation cannot be 
directly underlined, as the line from &\phantom{=} will be a little 
higher. I guess one might try &\underline{=\phantom{y}}, but I haven't 
tried this. If its works, it will be shorter.

I'll be interested in other solutions.


Sebastian Canagaratna
Department of Chemistry and Biochemistry
Ada, Ohio 45810