Answer
$1\times(1+1)+2\times(2+1)+3\times(3+1)+...+n\times(n+1)$
Work Step by Step
Starting with i=1, use n to compute the first few terms. Since we do not know what n is we make use of "..." so show that the same pattern is continued with the next j's until j=n, where it ends.
$1\times(1+1)+2\times(2+1)+3\times(3+1)+...+n\times(n+1)$