documentclass[a5paper]{article}
usepackage{amsmath}
begin{document}
Suppose we want to find out how much\
line-spacing is needed for math expressions.

With a verb|textstyle| expression like $tfrac{a}{n}$\
and something in-between just to fill space,\
and a verb|displaystyle| expression like $dfrac{a}{n}$\
the line spacings are evidently unequal.\
However, if we use a verb|smash| as in smash{$dfrac{a}{n}$}\
we will get the normal spacing, though\
there may be overlaps,!

For some special cases, like $a/n$ \
verb|raisebox| may give us the form smash{$dfrac{text{raisebox{-1ex}{$a$}}}
                            {text{raisebox{.7ex}{$n$}}}$}\
which may look perfect, but is by no\
means a simple general solution,!!
end{document}