\[\sum\]
means add, so that \[\sum x\]
meams 'add all the \[x\]
's.The summation symbol is often accompanied by indices top and bottom which indicate which
\[x\]
's to add.
\[\sum_{i=3}^7 x_i=x_3+x+4+x_5+x_6+x_7\]
\[\sum_{i=3}^7 i^2=3^2+4^2+5^2+6^2+7^2\]