Answer
$\sum_{k=1}^{20}k$
Work Step by Step
We need to compute summation formula for the
$1+2+3+.......+20$
We see that there are $20$ positive terms, take it as index $k$.
When $k=1$, the first term is $1$ and for $k=2$ , the second term is $2$. Thus, we write the summation formula as follows:
$1+2+3+\displaystyle ......+20=\sum_{k=1}^{20}k$