Answer
$20301$
Work Step by Step
$a_{1}+a_{2}+a_{3}\ldots +a_{n}=\dfrac {a_{1}+a_{n}}{2}\times n$
$d=a_{n+1}-a_{n}=9-5=5-1=4$
$a_{n}=a_{1}+\left( n-1\right) d\Rightarrow 401=1+\left( n-1\right) \times 4\Rightarrow \left( n-1\right) =\dfrac {401-1}{4}=100\Rightarrow n=101$
$1+5+9\ldots +401=\dfrac {1+401}{2}\times 101=20301$
