as far as I know, LyX doesn't know the dfrac macro. You can write into the preamble
\usepackage{amsmath} \let\frac\dfrac
then every frac is written as a \dfrac macro.
Splendid! It works! Just being curious: your solution affects all fractions; is there some way of having the two types of fractions in the same document?
sure, without the above two commands and \dfrac appears not in WYSIWIG in the math box.
On the other hand, you can use the default fraction, mark it with the mouse and then change the font style with the math panel to displaystyle.
Herbert
