Trouble using flalign for left aligned math equations

Question

I tried to left align some equations, but always get the wrong result

Code below

\documentclass[a4paper]{article}
\usepackage[fleqn]{amsmath}
\usepackage{showframe}
\begin{document}
\section*{Test}

\begin{flalign}
&&U_{i}&&= U_{\mathrm{h}i} + U_{\mathrm{r}i} \ ,  &&I_{i} &&= \frac{1}{Z_{\mathrm{L}i}}(U_{\mathrm{h}i} - U_{\mathrm{r}i})\\
&&U_{\mathrm{h}i}&&= \frac{U_i + Z_{\mathrm{L}i}I_{i}}{2} \ , &&U_{\mathrm{r}i} &&= \frac{U_i - Z_{\mathrm{L}i}I_{i}}{2}
\end{flalign}

\end{document}

What I would like to have:

Thank you all

@Werner I seached some method before. flalign align alignat I have all tried — sun0727, Oct 05 '18 at 17:28
why do you want to left align the left hand side of the =??? so making the space around the = asymmetrical? that looks very odd and quite unlike a normal left aligned equation setting in amsmath. — David Carlisle, Oct 05 '18 at 18:01
@DavidCarlisle May be not good for Quality. But it is true a trouble for me. In my opinion, & split the equation and each part should be left aligned — sun0727, Oct 05 '18 at 18:08
it makes it harder to read (and distracts the reader with an unusual layout) but it's your document and a free world, if that's what you what I see egreg and Werner have provided solutions:-) — David Carlisle, Oct 05 '18 at 18:35

Werner · Accepted Answer · 2018-10-05T17:56:58.287

2

You can just use flalign with & around the alignment relations and between each of the equations as well as a final & on each row:

\documentclass{article}

\usepackage{amsmath,eqparbox,xparse}

% https://tex.stackexchange.com/a/34412/5764
\makeatletter
\NewDocumentCommand{\eqmathbox}{o O{c} m}{%
  \IfValueTF{#1}
    {\def\eqmathbox@##1##2{\eqmakebox[#1][#2]{$##1##2$}}}
    {\def\eqmathbox@##1##2{\eqmakebox{$##1##2$}}}
  \mathpalette\eqmathbox@{#3}
}
\makeatother

\begin{document}

\begin{flalign}
              \eqmathbox[L1][l]{U_i} &= U_{\mathrm{h}i} + U_{\mathrm{r}i},  & 
              \eqmathbox[L2][l]{I_i} &= \frac{1}{Z_{\mathrm{L}i}}(U_{\mathrm{h}i} - U_{\mathrm{r}i}) & \\
  \eqmathbox[L1][l]{U_{\mathrm{h}i}} &= \frac{U_i + Z_{\mathrm{L}i}I_{i}}{2}, & 
  \eqmathbox[L2][l]{U_{\mathrm{r}i}} &= \frac{U_i - Z_{\mathrm{L}i}I_{i}}{2} &
\end{flalign}

\end{document}

The use of eqparbox supports alignment choices for elements within \eqmathbox[<tag>][<align>]{<stuff>}. All elements with the same <tag> will have the same maximum width where you can specify the individual <align>ment as needed (left in the above example).

An alternative without eqparbox:

\documentclass{article}

\usepackage{mathtools}

\begin{document}

\begin{flalign}
  \mathrlap{U_i}\phantom{U_{\mathrm{h}i}} &= U_{\mathrm{h}i} + U_{\mathrm{r}i},  & 
  \mathrlap{I_i}\phantom{U_{\mathrm{r}i}} &= \frac{1}{Z_{\mathrm{L}i}}(U_{\mathrm{h}i} - U_{\mathrm{r}i}) & \\
                          U_{\mathrm{h}i} &= \frac{U_i + Z_{\mathrm{L}i}I_{i}}{2}, & 
                          U_{\mathrm{r}i} &= \frac{U_i - Z_{\mathrm{L}i}I_{i}}{2} &
\end{flalign}

\end{document}

edited Oct 05 '18 at 17:56

answered Oct 05 '18 at 17:28

Werner

603,163

thanks for the answer, but I also need the equation begin from left, not centered – sun0727 Oct 05 '18 at 17:29
@sun0727: Like which of the following two setups: image Equations (1) & (2) or equations (3) & (4)? – Werner Oct 05 '18 at 17:34
like the second one, but Ui shoule also at the line begin, so the space betwenn Ui and = can be more. – sun0727 Oct 05 '18 at 17:37
thanks, I have uploaded a figure for what I want to have. – sun0727 Oct 05 '18 at 17:47
@sun0727: See the update. – Werner Oct 05 '18 at 17:57
very nice, but can you explain, why it is not so easy to approach, in my opinion, & split the equation, and each part just should be left aligned – sun0727 Oct 05 '18 at 18:13
@sun0727: The default behaviour is the have the LHS of the equation right-aligned and the RHS of the equation left-aligned. If you want to change that, you have to play around a bit. – Werner Oct 05 '18 at 18:14

egreg · Answer 2 · 2018-10-05T17:54:24.470

I can see no reason for the equation to start at the left margin. Anyway, here's the code

\documentclass[a4paper]{article}
\usepackage[fleqn]{amsmath}
\usepackage{showframe}

\begin{document}

\section*{Test}

\begin{flalign}
U_{i}           &= U_{\mathrm{h}i} + U_{\mathrm{r}i} ,  
  & I_{i} &= \frac{1}{Z_{\mathrm{L}i}}(U_{\mathrm{h}i} - U_{\mathrm{r}i}) && \\
U_{\mathrm{h}i} &= \frac{U_i + Z_{\mathrm{L}i}I_{i}}{2} ,
  & U_{\mathrm{r}i} &= \frac{U_i - Z_{\mathrm{L}i}I_{i}}{2} &&
\end{flalign}

\end{document}

With the standard align:

\documentclass[a4paper]{article}
\usepackage[fleqn]{amsmath}
\usepackage{showframe}

\begin{document}

\section*{Test}

\begin{align}
U_{i}           &= U_{\mathrm{h}i} + U_{\mathrm{r}i} ,  
  & I_{i} &= \frac{1}{Z_{\mathrm{L}i}}(U_{\mathrm{h}i} - U_{\mathrm{r}i}) \\
U_{\mathrm{h}i} &= \frac{U_i + Z_{\mathrm{L}i}I_{i}}{2} ,
  & U_{\mathrm{r}i} &= \frac{U_i - Z_{\mathrm{L}i}I_{i}}{2}
\end{align}

\end{document}

You get

which agrees with all other equations, since you're using fleqn.

If your aim is to align to the left U and I, then use a phantom. I took the occasion for simplifying your input by defining \rh, \rr and \rL. The macro \prh stands for “a phantom \rh”.

\documentclass[a4paper]{article}
\usepackage[fleqn]{amsmath}
\usepackage{showframe}

\newcommand{\rh}{\mathrm{h}}
\newcommand{\prh}{\hphantom{\rh}}
\newcommand{\rL}{\mathrm{L}}
\newcommand{\rr}{\mathrm{r}}

\begin{document}

\section*{Test}

\begin{align}
U_{i\prh}           &= U_{\rh i} + U_{\rr i} ,  
  & I_{i\prh} &= \frac{1}{Z_{\rL i}}(U_{\rh i} - U_{\rr i}) \\
U_{\rh i} &= \frac{U_i + Z_{\rL i}I_{i}}{2} ,
  & U_{\rh i} &= \frac{U_i - Z_{\rL i}I_{i}}{2}
\end{align}

\end{document}

Adapt to the flalign shown above, if you prefer.

@sun0727 I don't think it improves the quality. But I added it. — egreg, Oct 05 '18 at 17:55

Bernard · Answer 3 · 2018-10-06T08:19:26.183

The flalign environment requires only 5 = 2×3 – 1 ampersands since it has 3 groups of alignment.

Another possibility, to left align only some equations in your document; consists in nesting your equations in a fleqn environment, defined by nccmath. Furthermore, it can take an optional argument for the distance from the left margin at which the equation body begins (by default, it is 0pt). Demo:

\documentclass[a4paper]{article}
\usepackage{amsmath, nccmath}
\usepackage{showframe}
\renewcommand{\ShowFrameLinethickness}{0.3pt}

\begin{document}
\section*{Test}

\begin{flalign}
U_{i}&= U_{\mathrm{h}i} + U_{\mathrm{r}i} &I_{i} &= \frac{1}{Z_{\mathrm{L}i}}(U_{\mathrm{h}i} - U_{\mathrm{r}i}) & \\
U_{\mathrm{h}i}&= \frac{U_i + Z_{\mathrm{L}i}I_{i}}{2}&U_{\mathrm{r}i}&= \frac{U_i - Z_{\mathrm{L}i}I_{i}}{2}
\end{flalign}
\vspace{\baselineskip}
\begin{fleqn}[1em]
\begin{align}
U_{i}&= U_{\mathrm{h}i} + U_{\mathrm{r}i} &I_{i} &= \frac{1}{Z_{\mathrm{L}i}}(U_{\mathrm{h}i} - U_{\mathrm{r}i}) & \\
U_{\mathrm{h}i}&= \frac{U_i + Z_{\mathrm{L}i}I_{i}}{2}&U_{\mathrm{r}i}&= \frac{U_i - Z_{\mathrm{L}i}I_{i}}{2}
\end{align}
\end{fleqn}

\end{document}

Trouble using flalign for left aligned math equations

3 Answers3