The series is given as:
$$
\sum_{k=1}^n k
$$
This is an arithmetic series, not a geometric series. The formula for the sum of an arithmetic series is:
$$
S_n = \frac{n(a_1 + a_n)}{2}
$$
In this case, $a_1 = 1$ and $a_n = n$. So, we can plug these values into
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