Sum symbol
In Latex, to display a sum symbol, it is possible to use the command \sum.
\sum_{i=1}^{n}iThis gives :
$$\sum_{i=1}^{n}i$$
Sum multi-line
You can force the sum to be displayed on multiple lines like this :
\displaystyle \sum_{i=1}^{n}iThis gives :
$$\displaystyle \sum_{i=1}^{n}i$$
Sum mono-line
It is also possible to force the sum to be displayed in inline mode with the greek symbol \Sigma.
\Sigma_{i=1}^{n}iThis gives :
$$\Sigma_{i=1}^{n}i$$