Displaying four equations across two lines

Question

I want to write four equations over two lines within my Latex document. So, there would be one equation in the top-left, one in the top-right, one in the bottom-left, and one in the bottom-right. I then want to have a single Equation number to represent all four.

Here is what I have tried so far:

\begin{equation}
\small
\noindent\begin{minipage}{0.5\linewidth}
\begin{equation*}
a = x + 1 % top-left
\end{equation*}
\end{minipage}%
\begin{minipage}{0.5\linewidth}
\begin{equation*}
b = x + 2 % top-right
\end{equation*}
\end{minipage}
% Want to start a new line here....
\noindent\begin{minipage}{0.5\linewidth}
\begin{equation*}
c = x + 3 % bottom-left
\end{equation*}
\end{minipage}%
\begin{minipage}{0.5\linewidth}
\begin{equation*}
d = x + 4 % bottom-right
\end{equation*}
\end{minipage}
\end{equation}

However, this just places all four equations onto a single line. Instead, I want to be able to start a new line after the second equation. But using \\ or \newline does not seem to work here.

Any suggestions? Thanks!

Answer 1

Load amsmath and use one of these this codes. The second lets you control the horizontal skip between the two groups of equations:

\documentclass[10pt]{article}
 \usepackage{mathtools}

 \begin{document}

Layout with \verb|aligned|:
\begin{gather}
    \begin{aligned}
    a &= x + 1 % top-left
    & b &= x + 2 % top-right
    % Want to start a new line here...
    \\
    c &= x + 3 % bottom-left
    & d &= x + 4 % bottom-right
    \end{aligned}
    \\
    \begin{multlined}[b]
      a²b² + c² + d² =  (x + 1)² + (x + 2)² + (x + 3) ²+ (x + 4)²\\
      =4x² + 2x(1 + 2 + 3 + 4) + 1² + 2² + 3² + 4² = 4x² + 20x + 30
    \end{multlined}
\end{gather}
\bigskip

Layout with \verb|alignedat|:
\begin{gather}
    \begin{alignedat}{2}
    a &= x + 1 % top-left
    &\hspace{6em} b &= x + 2 % top-right
    % Want to start a new line here...
    \\
    c &= x + 3 % bottom-left
    & d &= x + 4 % bottom-right
    \end{alignedat} \\
a + b + c + d = (x + 1) + (x + 2) + (x + 3) + (x + 4) = 4x + 10
\end{gather}

\end{document}