split inside flalign
You should be aware that flalign
stands for full length align and not for flush left align.
As such, split
doesn't really make sense inside it.
You can emulate split
using aligned
.
\documentclass{article}
\usepackage{amsmath}
\usepackage{lipsum} % for mock text
\begin{document}
\lipsum[1][1-3]
\begin{flalign}
\begin{aligned}
a1 & = 3534534343 \\
a2 & = 2342342 \\
a3 & = 4564564 \\
a4 & = 5845645645864596464 \\
a5 & = 345354 \\
a6 & = 1
\end{aligned}&&
\end{flalign}
\lipsum[2][1-3]
\end{document}
On the other hand, I see no reason for some displays to be flush left and others centered.
It is not very clear what you like to achieve, but the following solution may help you:
\documentclass{article}
\usepackage{nccmath}
\usepackage{lipsum}
\begin{document}
\lipsum[11]
\begin{fleqn} % defined in nccmath
\begin{equation}
\begin{split}
a1 & = 3534534343 \\
a2 & = 2342342 \\
a3 & = 4564564 \\
a4 & = 5845645645864596464 \\
a5 & = 345354 \\
a6 & = 1
\end{split}
\end{equation}
\end{fleqn}
\lipsum[12]
\[
c^2 = a^2 + b^2
\]
\end{document}
Environment \begin{flegn} ... \end{fleqn}
(defined in nccmath
package) locally align equation to the left (as do amsmath
option fleqn
for whole document).