Question

Find the sum of first $8$ multiples of $3$.

Answer 1

First $8$ multiples of $3$-

$3,6,9,.....\text{upto 8 terms}$

The above series is in A.P. where,

First term $\left(a\right)=3$

Common difference $\left(d\right)=3$

No. of terms $\left(n\right)=8$

Sum of terms $\left(S_n\right)=?$

As we know that, in an A.P.,

$S_n=\frac{n}{2}[2a+(n-1)d]$

$\therefore S_8=\frac{8}{2}[2\times 3+(8-1)\times 3]$

$\Rightarrow S_8=4\times (6+21)$

$\Rightarrow S_8=4\times 27=108$