Question

Find the sum of first $15$ terms of the sequence whose $n^{th}$ term is $3+4n$.

• A. $525$
• B. $425$
• C. $495$
• D. $Noneofthes$

Answer 1

The correct option is A $525$

$n^{th}$ term = 3+4 n

$1^{st}$ term = 3+4(1)=7

$2^{nd}$ term = 3+4(2)=11

$3^{rd}$ term 3+4(3)=15

It is an AP with a=7, d=4

$S_{15}=\frac{15}{2}\left[2\right(7)+14(4\left)\right]$

$=15[7+28]$

$=525$