Originally Posted by **Liuxinhua** $\displaystyle \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}}$ |

When you "stack" fractions like this the font size gets a little small. Personally I'd prefer something like this:

$\displaystyle \frac{1}{\sqrt{1 - (v/c)^2 }}$

There is also a way to force the LaTeX into changing the font size inside the bottom fraction. I don't recall if it was simply resizing the letters or if there was another command involved.

-Dan