Question

Find the sum of the first $15$ terms of the sequences having $n$th term as
$x_n=6-n$

• A. $30$
• B. $-30$
• C. $20$
• D. $-20$

Answer 1

Answer: (B)

$-30$

Formula,

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

Given,

$a_n=6-n$

$a_1=6-1=5$

$a_2=6-2=4$

$d=4-5=-1$

$S_{15}$= $\frac{15}{2}\left[2\right(5)+(15-1)\times (-1\left)\right]$

$\therefore S_{15}=-30$