# Aligning multiline equation to the left with only one equation number

My problem is:

I would like to align a splitted equation to the left of the page with only one equation number.

If I use split then the equations won't align to the left and if I use flalign then I would get two equation numbers

Using split

$$\begin{split} F\{j(t)\}=&-\tfrac{j_{max}}{2\pi jf}e^{-2\pi jf\frac{a_{max}}{j_{max}}}+\tfrac{j_{max}}{2\pi jf}+\tfrac{j_{max}}{2\pi jf}e^{-2\pi jf(\frac{v_{max}}{a_{max}}+\frac{a_{max}}{j_{max}})} \\ &-\tfrac{j_{max}}{2\pi jf}e^{-2\pi jf\frac{v_{max}}{a_{max}}} \end{split}$$


Using flalign

\begin{flalign}
F\{j(t)\}=&-\tfrac{j_{max}}{2\pi jf}e^{-2\pi jf\frac{a_{max}}{j_{max}}}+\tfrac{j_{max}}{2\pi jf}+\tfrac{j_{max}}{2\pi jf}e^{-2\pi jf(\frac{v_{max}}{a_{max}}+\frac{a_{max}}{j_{max}})} &\\
&-\tfrac{j_{max}}{2\pi jf}e^{-2\pi jf\frac{v_{max}}{a_{max}}}
\end{flalign}


So the two equations in split are centered and I would like them to the left. And the two equations in aligned are to the left but have two equation numbers (see immage). How do I solve this?

You can put a whole split inside the first field of flalign, or better used aligned here:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{flalign}
\begin{aligned}
F\{j(t)\}=&-\tfrac{j_{\max}}{2\pi jf}e^{-2\pi jf\frac{a_{\max}}{j_{\max}}}+\tfrac{j_{\max}}{2\pi jf}+\tfrac{j_{\max}}{2\pi jf}e^{-2\pi jf(\frac{v_{\max}}{a_{\max}}+\frac{a_{\max}}{j_{\max}})} \\
&-\tfrac{j_{\max}}{2\pi jf}e^{-2\pi jf\frac{v_{\max}}{a_{\max}}}
\end{aligned}&&&
\end{flalign}

\end{document}


The aligned building block can be used anywhere inside other math constructions. split is more restricted.

You also can have one of these alignments. I used the fleqn environment from nccmath, which sets temporarily the fleqn option from amsmath. Also nccmath defines medium-sized fractions, which look better, in my opinion, in the context of a displayed equation:

\documentclass{article}
\usepackage{geometry}
\usepackage{mathtools, nccmath}

\begin{document}

\begin{fleqn}
\begin{aligned} F\{j(t)\}=-\mfrac{j_{\max}}{2\pi jf}e^{-2\pi jf\tfrac{a_{\max}}{j_{\max}}}+\mfrac{j_{\max}}{2\pi jf}+\mfrac{j_{\max}}{2\pi jf}e^{-2\pi jf\begin{pmatrix}\frac{v_{\max}}{a_{\max}}+\frac{a_{\max}}{j_{\max}}\end{pmatrix}} \\[-1ex] -\mfrac{j_{\max}}{2\pi jf}e^{-2\pi jf\tfrac{v_{\max}}{a_{\max}}} \end{aligned}%

\begin{aligned} F\{j(t)\}=-\mfrac{j_{\max}}{2\pi jf}e^{-2\pi jf\tfrac{a_{\max}}{j_{\max}}}+\mfrac{j_{\max}}{2\pi jf} & +\mfrac{j_{\max}}{2\pi jf}e^{-2\pi jf\begin{pmatrix}\frac{v_{\max}}{a_{\max}}+\frac{a_{\max}}{j_{\max}}\end{pmatrix}} \\[-0.5ex] & -\mfrac{j_{\max}}{2\pi jf}e^{-2\pi jf\tfrac{v_{\max}}{a_{\max}}} \end{aligned}
\end{fleqn}

\end{document}


An alternative with TABstacks:

\documentclass{article}
\usepackage{amsmath,tabstackengine}
\begin{document}
\begin{flalign}
{\setstackgap{L}{21pt}\ensurestackMath{\tabbedCenterstack[l]{
F\{j(t)\}=&{}-\tfrac{j_{max}}{2\pi jf}e^{-2\pi jf\frac{a_{max}}{j_{max}}}+\tfrac{j_{max}}{2\pi jf}+\tfrac{j_{max}}{2\pi jf}e^{-2\pi jf(\frac{v_{max}}{a_{max}}+\frac{a_{max}}{j_{max}})} \\
&{}-\tfrac{j_{max}}{2\pi jf}e^{-2\pi jf\frac{v_{max}}{a_{max}}}
}}}&&
\end{flalign}
\end{document}