Aligning multiline equation to the left with only one equation number

Question

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{equation}
\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}
\end{equation}

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?

Answer 1

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.

Answer 2

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{equation}
  \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}%
\end{equation}

\begin{equation}
  \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{equation}
\end{fleqn}

\end{document} 

Answer 3

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}