How to add a horizontal rule in align* environment between equations

Question

I am trying to add a horizontal line between aligned equations.

I have tried using \rule command, but then the problem is that it itself takes that command as an equation, since I am using \\ before and after this command, otherwise it is not aligning the equations correctly. With these, it uses unnecessary spaces above and below the horizontal rule. I want the output to be such that the equations are aligned as expected from the align* environment, with a horizontal line 'externally inserted' between two equations.

\begin{align*}
    y_2\times (y_2''+Py_2'+Qy_2) &=0\\
    -y_1\times (y_1''+Py_1'+Qy_1) &=0\\
    \rule{4.7cm}{1pt}&\rule{1cm}{1pt}\\
    y_1y_2''-y_1''y_2+P(y_1y_2'-y_1'y_2) &= 0\\
\end{align*}

The above is what I wrote, but it shows too much space above and below the horizontal rule. Even if I shift this rule 3mm upwards, the space below the rule increases and the problem remains same. Is there any way to add a horizontal rule whose space below and above can also be controlled?

Please help.

Answer 1

With the help of an array instead of align*:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\setlength{\arraycolsep}{0pt}
\begin{array}{rl}
     y_2\times (y_2''+Py_2'+Qy_2) &{}=0\\
    -y_1\times (y_1''+Py_1'+Qy_1) &{}=0\\
    \hline
     y_1y_2''-y_1''y_2+P(y_1y_2'-y_1'y_2) &{}= 0\\
\end{array}
\]
\end{document}

Answer 2

You can use aligned and \midrule from booktabs.

\documentclass{article}
\usepackage{amsmath,booktabs}

\begin{document}

\begin{equation*}
\begin{aligned}
    y_2\times (y_2''+Py_2'+Qy_2) &=0\\
    -y_1\times (y_1''+Py_1'+Qy_1) &=0\\
    \midrule
    y_1y_2''-y_1''y_2+P(y_1y_2'-y_1'y_2) &= 0\\
\end{aligned}
\end{equation*}

\end{document}