Answer
$$\require{cancel}
\sum_{k=1}^{40}k = 820$$
Work Step by Step
RECALL:
$$\sum_{i=1}^{n}k = \dfrac{n(n+1)}{2}$$
Use the rule above to obtain:
$$\require{cancel}
\sum_{k=1}^{40}k = \dfrac{40(41)}{2}=820$$
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