# How to write this long equation?

\begin{eqnarray*}
\vec{E}_{tot}=q\cdot k_{b}\cdot \dfrac{r}{r^3}\left\lgroup
\dfrac{\hat{r}-\left(\dfrac{d}{2\cdot r}\right)\hat{d}}{\left(1+\left(\dfrac{d}{2\cdot r}\right)^2-\left(\dfrac{d}{ r}\right)\hat{r}\cdot\hat{d}\cdot cos (\theta)\right)^{3/2}}-\dfrac{\hat{r}+\left(\dfrac{d}{2\cdot r}\right)\hat{d}}{\left(1+\left(\dfrac{d}{2\cdot r}\right)^2+\left(\dfrac{d}{ r}\right)\cdot\hat{r}\cdot\hat{d}\cdot cos(\theta)\right)^{3/2}}
\right\rgroup

\end{eqnarray*}

• Welcome to TeX SX! Could you post a full compilable code? May 25 '17 at 16:33

This is a case for multline:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{multline*}
\vec{E}_{\mathrm{tot}}=
q\cdot k_{b}\cdot \dfrac{r}{r^3}
\left\lgroup
\frac{\hat{r}-\left(\dfrac{d}{2\cdot r}\right)\hat{d}}
{\biggl(1+\left(\dfrac{d}{2\cdot r}\right)^2-
\left(\dfrac{d}{r}\right)\hat{r}\cdot\hat{d}\cdot\cos(\theta)
\biggr)^{3/2}}
\right.
\\
\left.
{}-
\frac{\hat{r}+\left(\dfrac{d}{2\cdot r}\right)\hat{d}}
{\biggl(1+\left(\dfrac{d}{2\cdot r}\right)^2+
\left(\dfrac{d}{r}\right)\cdot\hat{r}\cdot\hat{d}\cdot\cos(\theta)
\biggr)^{3/2}}
\right\rgroup
\end{multline*}

\end{document}


I changed a couple of \left-\right pairs so to keep the outer parentheses a bit smaller.

Second, by getting rid of all \cdot directives and replacing most \left and \right directives with smaller sizing opeators \Big and \bigg), and assuming normal-width margins, it's easily possible to write the entire equation on a single line.

Separately, you may want to look into not using displaymath-mode fractional notation. The result look is shown in the second equation of the following screenshot.

\documentclass[10pt,a4paper]{article}
\usepackage{amsmath,mleftright}
\newcommand\ddfrac[2]{\frac{\displaystyle #1}{\displaystyle #2}}
\usepackage[margin=2.5cm]{geometry} % choose page margins suitable
\begin{document}

$\vec{E}_{\mathrm{tot}}=q k_{b}^{} \frac{r}{r^3} \left( \ddfrac{\hat{r}-\Bigl(\frac{d}{2r}\Bigr)\hat{d}}{% \biggl(1+\Bigl(\frac{d}{2r}\Bigr)^{\!2} -\Bigl(\frac{d}{r}\Bigr)\hat{r}\hat{d}\cos(\theta)\biggr)^{\!3/2}} -\ddfrac{\hat{r}+\Bigl(\frac{d}{2r}\Bigr)\hat{d}}{% \biggl(1+\Bigl(\frac{d}{2r}\Bigr)^{\!2} +\Bigl(\frac{d}{r}\Bigr)\hat{r}\hat{d}\cos(\theta)\biggr)^{\!3/2}} \right)$

\bigskip
%% and now without displaystyle fractions
$\vec{E}_{\mathrm{tot}}=q k_{b}^{} \tfrac{r}{r^{3\mathstrut}} \mleft[ \dfrac{\hat{r}-\frac{d}{2r}\hat{d}}{% \bigl[1+\frac{d^2}{4r^{2\mathstrut}} -\frac{d}{r}\hat{r}\hat{d}\cos(\theta)\bigr]^{3/2}} -\dfrac{\hat{r}+\frac{d}{2r}\hat{d}}{% \bigl[1+\frac{d^2}{4r^{2\mathstrut}} +\frac{d}{r}\hat{r}\hat{d}\cos(\theta)\bigr]^{3/2}} \mright]$

\end{document}


Since it is one equation, I would rather use the multline(*) environment in this case.

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{multline*}
\vec{E}_{tot} = q\cdot k_{b}\cdot \dfrac{r}{r^3}\left\lgroup
\dfrac{\hat{r}-\left(\dfrac{d}{2\cdot r}\right)\hat{d}}{\left(1+
\left(\dfrac{d}{2\cdot r}\right)^2-\left(\dfrac{d}{ r}\right)\hat{r}
\cdot\hat{d}\cdot \cos (\theta)\right)^{3/2}}\right.\\
- \left.\dfrac{\hat{r}+\left(\dfrac{d}{2\cdot r}\right)\hat{d}}
{\left(1+\left(\dfrac{d}{2\cdot r}\right)^2
+\left(\dfrac{d}{ r}\right)\cdot\hat{r}\cdot\hat{d}\cdot \cos(\theta)
\right)^{3/2}}\right\rgroup
\end{multline*}
\end{document}


Your code is incorrect, since you use a blank line just before the end of eqnarray*. Just remove that, then it works.

\documentclass{article}
\usepackage{amsmath,amsfonts,amssymb}

\begin{document}
\begin{eqnarray*}
\vec{E}_{tot}=q\cdot k_{b}\cdot \dfrac{r}{r^3}\left\lgroup
\dfrac{\hat{r}-\left(\dfrac{d}{2\cdot r}\right)\hat{d}}{\left(1+\left(\dfrac{d}{2\cdot r}\right)^2-\left(\dfrac{d}{ r}\right)\hat{r}\cdot\hat{d}\cdot cos (\theta)\right)^{3/2}}-\dfrac{\hat{r}+\left(\dfrac{d}{2\cdot r}\right)\hat{d}}{\left(1+\left(\dfrac{d}{2\cdot r}\right)^2+\left(\dfrac{d}{ r}\right)\cdot\hat{r}\cdot\hat{d}\cdot cos(\theta)\right)^{3/2}}
\right\rgroup
\end{eqnarray*}
\end{document}


Update: As alsways it is a bad idea to use eqnarray* if there are such great environments as, f.i., align*, which could btw help you break those equations.

\documentclass{article}
\usepackage{amsmath,amsfonts,amssymb}

\begin{document}
\begin{align*}
\vec{E}_{tot}&=q\cdot k_{b}\cdot \dfrac{r}{r^3}\left\lgroup
\dfrac{\hat{r}-\left(\dfrac{d}{2\cdot r}\right)\hat{d}}{\left(1+\left(\dfrac{d}{2\cdot r}\right)^2-\left(\dfrac{d}{ r}\right)\hat{r}\cdot\hat{d}\cdot \cos (\theta)\right)^{3/2}}\right.\\
&- \left.\dfrac{\hat{r}+\left(\dfrac{d}{2\cdot r}\right)\hat{d}}{\left(1+\left(\dfrac{d}{2\cdot r}\right)^2+\left(\dfrac{d}{ r}\right)\cdot\hat{r}\cdot\hat{d}\cdot \cos(\theta)\right)^{3/2}}\right\rgroup
\end{align*}
\end{document}

• &= not =& (see the (no) space around the = May 25 '17 at 18:31

It can perfectly fit in a single line, if you remove the \cdots, which I think are unnecessary, and a number of useless parentheses. Unrelated to the problem at hand, I replaced \vec, which isn't so nice with capital letters, with \vv from esvect:

\documentclass{article}

\usepackage{mathtools}
\usepackage{esvect}
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.3pt}

\begin{document}

\begin{equation*}
\vv*{E}{\text{tot}} =q k_{b}\, \frac{r}{r^3}\mkern-6mu%
\left\lgroup
\frac{\hat{r}-\dfrac{d}{2 r}\,\hat{d}}{\biggl(1+\biggl(\dfrac{d}{2 r}\biggr)^{\!2}\!-\dfrac{d}{ r}\,\hat{r}\hat{d}\,\cos \theta\biggr)^{\!\tfrac{3}{2}}}%
-\frac{\hat{r}+ \dfrac{d}{2 r}\,\hat{d}}{\biggl(1+\left(\dfrac{d}{2 r}\right)^{\!2}\!+\dfrac{d}{ r}\,\hat{r}\hat{d}\, \cos \theta \biggr)^{\!\tfrac{3}{2}}}
\right\rgroup
\end{equation*}

\end{document}


• The use of \! in \biggr)^{\!2} makes a big difference. IMO it improves the output too, by more closely associating the power with the bracket May 26 '17 at 9:12