Question

The sum of first $10$ terms of the A.P. $-2,1,4,7,$ is:

• A. $115$
• B. $230$
• C. $1000$
• D. $620$

Answer 1

Answer: (A)

$115$

First term of the A.P. is $a=-2$

Common difference, $d=a_2-a_1$

$=1-(-2)=3$

Sum of first $n$ terms of A.P. is

$S_n=\frac{n}{2}\{2a_1+(n-1\left)d\right\}$

Sum of first $10$ terms of given A.P. is

$S_{10}=\frac{10}{2}\left\{2\right(-2)+9(3\left)\right\}$

$=5\times 23$

$=115$