displaying fractions in line in equation mode

Question

I am new to latex and am currently preparing a document. I want to write the following equation. I have tried the following in latex. I am not able to get the big 'slash' for the inline fraction. How do I do it?

\phi_{i}=x_{i}+\sum_{k\neq1}x_{k}\left[\frac{5}{3}\frac{1}{A_{ik}^{*}}+\frac{M_{k}}{M_{i}}\right]/\left[1+\frac{M_k}{M_i}\right]{x}\left[F_{ik}+B_{ik}\sqrt{\frac{\eta_i}{\eta_k}}\left(\frac{M_{k}}{M_{i}}\right)^{1/4}\right]^2/\sqrt{8\left(1+\frac{M_i}{M_k}\right)}

Answer 1

If your space constraints are not strict, like two column typesetting, you can use \medmath from the nccmath package: this reduces ambiguities in the formula.

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{nccmath}

\begin{document}
\begin{equation*}
\phi_{i} = x_{i}+\sum_{k\neq i}x_{k}
\medmath{
  \frac{\displaystyle
    \frac{5}{3}\frac{1}{A_{ik}^{*}}+\frac{M_{k}}{M_{i}}
  }{\displaystyle
    1+\frac{M_k\mathstrut}{M_i}
  }
  \,
  \frac{\displaystyle
    \Bigl(
      F_{ik}+B_{ik}\sqrt{\frac{\eta_i}{\eta_k}}
      \Bigl(\frac{M_{k}}{M_{i}}\Bigr)^{1/4}
    \Bigr)^2
  }{\displaystyle
    \sqrt{8\left(1+\frac{M_i}{M_k}\right)}
  }
}% end of \medmath
\end{equation*}

\end{document}

Actually, a comparison with the slashed version shows the space is less with the above version.

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{nccmath}

\begin{document}

\begin{equation*}
\begin{split}
\phi_{i} ={}& x_{i}+\sum_{k\neq i}x_{k}
  \biggl[
    \frac{5}{3}\frac{1}{A_{ik}^{*}}+\frac{M_{k}}{M_{i}}
  \biggr]
  \bigg/
  \biggl[
    1+\frac{M_k}{M_i}
  \biggr]
\\
&\times
  \biggl[
    F_{ik}+B_{ik}\sqrt{\frac{\eta_i}{\eta_k}}\left(\frac{M_{k}}{M_{i}}\right)^{\!1/4}
  \biggr]^2
  \bigg/
  \biggl[
    \sqrt{8\left(1+\frac{M_i}{M_k}\right)}
  \biggr]
\end{split}
\end{equation*}

\begin{equation*}
\phi_{i} = x_{i}+\sum_{k\neq i}x_{k}
\medmath{
  \frac{\displaystyle
    \frac{5}{3}\frac{1}{A_{ik}^{*}}+\frac{M_{k}}{M_{i}}
  }{\displaystyle
    1+\frac{M_k\mathstrut}{M_i}
  }
  \,
  \frac{\displaystyle
    \Bigl(
      F_{ik}+B_{ik}\sqrt{\frac{\eta_i}{\eta_k}}
      \Bigl(\frac{M_{k}}{M_{i}}\Bigr)^{1/4}
    \Bigr)^2
  }{\displaystyle
    \sqrt{8\left(1+\frac{M_i}{M_k}\right)}
  }
}% end of \medmath
\end{equation*}

\end{document}

You can judge what's the clearer way to display your formula.

Answer 2

I would indent the second line to the right, so that the opening large square brackets are aligned, and I would add an additional pair of "fences" to make the structure of the equation more immediately obvious to the readers. Use \biggl[, \biggr, and \biggm to size the parentheses, brackets, and division symbols.

\documentclass{article}
\usepackage{amsmath} % for 'align*' env.
\begin{document}
\begin{align*}
\phi_i =x_i+\sum_{k\neq1}x_k \Biggl\{ 
&\biggl[\frac{5}{3} \frac{1}{A_{ik}^{*}} +\frac{M_k}{M_i}\biggr] \biggm/ 
\biggl[1+\frac{M_k}{M_i} \biggr]\\
\times&\biggl[F_{ik}+B_{ik}\sqrt{\frac{\eta_i}{\eta_k}} \biggl(\frac{M_k}{M_i}\biggr)^{\!\!1/4}\,\biggr]^2 \!\! \biggm/ 
\biggl[\sqrt{8}\biggl(1+\frac{M_i}{M_k}\biggr)^{\!\!1/2}\,\biggr] \Biggr\}
\end{align*}
\end{document}

Answer 3

Please check the modified tagging:

\phi_{i}=x_{i}+\sum_{k\neq1}x_{k}\left[\frac{5}{3}\frac{1}{A_{ik}^{*}}+\frac{M_{k}}{M_{i}}\right]\Bigg/\left[1+\frac{M_k}{M_i}\right]{x}\left[F_{ik}+B_{ik}\sqrt{\frac{\eta_i}{\eta_k}}\left(\frac{M_{k}}{M_{i}}\right)^{1/4}\right]^2\Bigg/\sqrt{8\left(1+\frac{M_i}{M_k}\right)}

Answer 4

\documentclass[12pt]{article}
\usepackage{amsmath}

\begin{document}
\[
\begin{split}
\phi_{i} & = x_{i}+\sum_{k\neq1}x_{k}
            \left[\frac{5}{3}\frac{1}{A_{ik}^{*}}+\frac{M_{k}}{M_{i}}\middle]
                        \middle/
            \middle[1+\frac{M_k}{M_i}\right]    \\ 
         & \times
            \left[F_{ik}+B_{ik}\sqrt{\frac{\eta_i}{\eta_k}}
            \left(\frac{M_{k}}{M_{i}}\right)^{1/4}\right]^2     
                        \left/
                \sqrt{8\left(1+\frac{M_i}{M_k}\right)}\right.
\end{split}
\]
\end{document}