Question

Find the sum of $20$ terms of the AP $1,4,7,10,.....$.

Answer 1

Given AP is $1,4,7,10,.....$

In given AP first term $a=1$, common difference $d=3$

Since, $S_n=\frac{n}{2}[2a+(n-1\left)d\right]$ ....... sum of $n$ terms

Put $n=20$ for sum of first $20$ terms

$S_{20}=\frac{20}{2}\left[2\right(1)+(20-1\left)3\right]$
$S_{20}=10[2+57]$
$S_{20}=590$