\bar{x} = \frac{\sum_{i=1}^n x_i}{n}

[Jacobo Myerston]

Hi,

I want to write the classical  statistic mean  with  \bar{x} = 
\frac{\sum_{i=1}^n  x_i}{n}, but that doesn't display like I 
expected. When I introduce \frac the subscript n  and _{i=1} display 
on the side of the sum and not above and under the sum. Is there 
another way to do that?
--