#### Answer

$S_{400}=80200$

#### Work Step by Step

The first n term of an arithmetic sequence can be calculated as:
$S_n=\frac{n}{2}(a_1+a_n)$
We have to find the sum of the first 400 natural numbers.
$a_1=1$
$a_{400}=400$
Therefore we can substitute in the formula of the sum, and we get:
$S_{400}=\frac{400}{2}(1+400))=200\times401=80200$