How to write sum in the equations

The easiest way to write a sum of an expression (i=[1 .. n]) in equations is

$\sum_{i=1}^n expression$

Note that in inline code this will put lower and upper bound next to the sum sign, not over and under. To  display lower and upper bound under and over the sum sign use the \displaystyle command before the sum.

To specify directly that the upper and lower bound are over and under the sum symbol, simply write

$\sum\limits_{i=1}^n expression$