Question

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

Answer 1

Let $a$ be the first term and $d$ be the common difference of the given A.P.

Then, we have $a=1$ and $d=3$

we have to find sum of $20$ terms of the given A.P.

Putting $a=1,d=3,n=20$ in $S_n=\frac{n}{2}\{2a+(n-1\left)d\right\}$, we get

$S_{20}=\frac{20}{2}\{2\times 1+(20-1)\times 3\}=10\times 59=590$