Question

Find the sum of the first $8$ terms of an arithmetic sequence whose first three terms are as follows : $7,10.5,14,.....$

• A. $143.5$
• B. $154$
• C. $162.5$
• D. $168$
• E. $176.5$

Answer 1

The correct option is B $154$

Sum of first $n^{th}$ term of an AP=$S_n=\frac{n}{2}[2a+(n-1\left)d\right]$

Where $n=$ no.of tern$=8$
$d=$ common difference between each term$=3.5$
$a=$ first term$=7$

$\therefore S_8=\frac{8}{2}[2\times 7+(8-1\left)3.5\right]$

$\Rightarrow S_8=\frac{8}{2}[14+7\times 3.5]$

$\Rightarrow S_8=4[14+24.5]$

$\Rightarrow S_8=4\times 38.5$

$\Rightarrow S_8=154.0$