Dots can be based on the surrounding text, if there is no right hand end to the dots then the type must be specified.
$x_1, x_2, \dots x_n$, $x_1 + x_2 + \dots + x_n$, $x_1,x_2, \dotsc$, $x_1 + x_2 + \dotsb$, $x_1 \cdot x_2 \dotsm$, $\iint \dotsi$
$x_1, x_2, \dots x_ n$, $x_1 + x_2 + \dots + x_ n$, $x_1,x_2, \dotsc $, $x_1 + x_2 + \dotsb $, $x_1 \cdot x_2 \dotsm $, $\iint \dotsi $