fractionsmath-modetext;
I know exactly how to generate it, but I want a more efficient way. Anybody can help?
\begin{equation}
\nonumber
n=\frac{\mathrm{maximum \ fraction \ of \ the \ river \ that \ a \ boat \ can \ traverse \ in \ a \ single \ day}}{\mathrm{distance \ between \ campsites \ as a \ fraction \ of \ total \ river \ length}},
\end{equation}
You should load the amsmath package and use \text{...}:
\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\[
\frac{ax^2+bx+c}{\sin^2y+\cos^2z} \qquad
\frac{\text{minaccia/vulnerabilit\`a}}{\text{contromisure}}
\]
\end{document}
You can also use \mbox{...} or \textrm{...}, but these won't scale when used inside exponents the way \text{...} does.
You can use \cfrac for nested fractions. However this makes the ‘upper’ denominators too close from the main fraction line, so one can compensate adding a phantom letter with descenders. No compensation required for the ‘lower’ numerators if you use \cfrac (not \dfrac).
Another possibility would use the \mfrac(medium-sized fraction – 80 % of \displaystyle) command from nccmath, so the smaller fractions are still readable. Here is an example of both methods:
\documentclass{article}
\usepackage{amsmath, nccmath}
\begin{document}
\begin{align*}
\frac{\sin(a+b)}{\cos(a+b)}&=\frac{\sin a \cos b + \cos a \sin b}{\cos a \cos b - \sin a \sin b}
\\
\tan(a+b)&=\frac{\sin a \cos b + \cos a \sin b}{\cos a \cos b - \sin a \sin b}
\\[0.5ex]
&=\frac{\cfrac{\sin a \cos b}{\cos a \cos b} + \cfrac{\cos a \sin b}{\cos a \cos b\vphantom{g}}}{\cfrac{\cos a \cos b}{\cos a \cos b} - \cfrac{\sin a \sin b}{\cos a \cos b}}
\\[0.5ex]
&=\frac{\cfrac{\sin a}{\cos a} + \cfrac{\sin b}{\cos b \vphantom{g}}}{1 - \cfrac{\sin a \sin b}{\cos a \cos b}}
\\[0.5ex]
&=\frac{\tan a +\tan b }{1 - \tan a \tan b}
\end{align*}\medskip
\begin{align*}
\frac{\sin(a+b)}{\cos(a+b)}&=\frac{\sin a \cos b + \cos a \sin b}{\cos a \cos b - \sin a \sin b}
\\
\tan(a+b)&=\frac{\sin a \cos b + \cos a \sin b}{\cos a \cos b - \sin a \sin b}
\\[0.5ex]
&=\frac{\mfrac{\sin a \cos b}{\cos a \cos b} + \mfrac{\cos a \sin b}{\cos a \cos b\vphantom{g}}}{\mfrac{\cos a \cos b}{\cos a \cos b} - \mfrac{\sin a \sin b}{\cos a \cos b}}
\\[0.5ex]
&=\frac{\mfrac{\sin a}{\cos a} + \mfrac{\sin b}{\cos b\vphantom{g}}}{1 - \mfrac{\sin a \sin b}{\cos a \cos b}}
\\[0.5ex]
&=\frac{\tan a +\tan b }{1 - \tan a \tan b}
\end{align*}
\end{document}
Best Answer
If this is really needed, then use
\text{foo foo}instead of\mathrm{foo\ foo}. The\textcommand is designed to display, well, text in a math environment, however it should be restricted to short text, in my point of view.