[Tex/LaTex] Fractions with `\dfrac` cause compile error

errorsfractionslyx

I'm new to LyX. I'm writing my next homework with it to give it a try. My example code is below:

% Preview source code

%% LyX 2.0.5.1 created this file.  For more info, see http://www.lyx.org/.
%% Do not edit unless you really know what you are doing.
\documentclass[english]{article}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{babel}
\begin{document}
This is an expression with dfrac:

\[ f(x) = \dfrac{2}{x} \]
\end{document}

Under another question, they offered me to use \dfrac fractions for a better view. However, using dfrac causes compiler error like below:

Compiler Error

I thought this error with dfrac may be due to not using the amsmath package. I went to Menu > Document > Settings > Math Options, I unchecked Use AMS math package automatically and I checked Use AMS math package. LyX added \usepackage{amsmath} and \usepackage{amssymb} lines into the source code. Now when I try to preview my article in PDF format, it gives LaTeX Error: File 'amsmath.sty' not found. error.

I used to use online Latex editors, then copy screen shot of their output and paste it into MS Word. It was so simple doing that way. Now all these packages, uneditable source code, and all these complexity for writing a simple fraction is making me too frustrated. I have started to question if I did the right thing with switching to LyX. Because those online editors perfectly displaying that dfrac fraction in my example.

How do I write a simple dfrac fraction after opening a new article from Menu > File > New in a fresh LyX 2.0.5.1 installation?

Best Answer

go to documents->settings->math-> enable amsmath

Fractions

I'd just use the normal \frac or \tfrac. For display style maths they still look the best. I would use \nicefrac (or similar) only (if at all) in text maths.

Code

\documentclass{article}
\usepackage{amsmath}
\def\frstfrac{\frac{2}{\sqrt{2y+1}\sqrt{2 \pi}}} % just a shortcut
\begin{document}
\begin{align*}
  &= \frstfrac       \exp \left(            -y - \frac{1}{2}\right)  && \forall y \in  \left[- \frac{1}{2}, \infty\right) \\
  &= \frstfrac \cdot  e^{                   -y - \frac{1}{2}}        && \forall y \in  \left[-\tfrac{1}{2}, \infty\right) \\
  &= \frstfrac \cdot  e^{           -  \left(y + \frac{1}{2}\right)} && \forall y \in  \left[-\tfrac{1}{2}, \infty\right) \\
  &= \frstfrac \cdot  e^{\textstyle -\!\left(y + \frac{1}{2}\right)} && \forall y \in  \left[-\tfrac{1}{2}, \infty\right) \\
  &= \frstfrac       \exp \left(            -y - 1/2 \right)         && \forall y \in  \left[-       1/ 2 , \infty\right)
\end{align*}
\end{document}

Output

$different outputs of the fraction$

`\smash`

Also, consider using \sqrt{\smash[b]{2y+1}}\sqrt{\smash[b]{2 \pi}} in your denominator for better roots.

To quote from the amsmath documentation:

With the amsmath package \smash has optional arguments t and b, because occasionally it is advantageous to be able to “smash” only the top or only the bottom of something while retaining the natural depth or height.

Compare:

Code

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
  \frac{2}{\sqrt{\smash[b]{2y+1}}\sqrt{\smash[b]{2 \pi}}} = \frac{2}{\sqrt{2y+1}\sqrt{2 \pi}}
\end{equation*}
\end{document}

Output

smash example

The top bar of the root is aligned different and even the lower point is not on equal heights.

LaTeX Companion

The LaTeX Companion has a very elaborated text about \smash and its uses. The first two examples (licensed under the LPPL) are shown here. Take a close look at the root's lower end and the top bar.

Code

\documentclass{article}
\usepackage{amsmath}
\begin{document}

$ \sqrt{x} + \sqrt{y}            + \sqrt{z} $ \par
$ \sqrt{x} + \sqrt{\mathstrut y} + \sqrt{z} $ \par
$ \sqrt{x} + \sqrt{\smash{y}}    + \sqrt{z} $ \par
$ \sqrt{x} + \sqrt{\smash[b]{y}} + \sqrt{z} $

\[
   \sqrt{ \frac{a+b}{         x_j   } } \quad
   \sqrt{ \frac{a+b}{  \smash{x_j  }} } \quad
   \sqrt{ \frac{a+b}{{}\smash{x_j  }} } \quad
   \sqrt{ \frac{a+b}{  \smash{x_j+b}} }
\]
\end{document}

Output

square root example $fraction example$

[Tex/LaTex] Undefined control sequence on legitimate LyX macro

As egreg mentions in a comment, the reason you get the error is that neither the amsfonts nor amssymb package, where \mathbb is defined, is loaded. To force LyX to add one of these you can do one of two things:

Go to Document --> Settings --> LaTeX preamble and add there either
```
\usepackage{amssymb}
```
or
```
\usepackage{amsfonts}
```
Go to Document --> Settings --> Math options. Uncheck Use AMS math package automatically and check Use AMS math package, as in the screenshot below. LyX will now have added
```
\usepackage{amsmath}
\usepackage{amssymb}
```
to the document preamble.

The first of these, the automatic one, adds these packages only if you use some of their features through the LyX GUI. E.g. if you do Insert --> Math --> AMS Align, the amsmath package will be added, and if you e.g. add some text with Blackboard bold via the math font button on the toolbar -- -- then amssymb will be added.

Best Answer

Related Solutions

[Tex/LaTex] issues with fractions

Fractions

Code

Output

\smash

Code

Output

LaTeX Companion

Code

Output

[Tex/LaTex] Undefined control sequence on legitimate LyX macro

Related Question

`\smash`