Question

The sum of $20$ terms of an A.P. whose nth term $4n-1$ is :

• A. $820$
• B. $79$
• C. $80n-1$
• D. $80n+1$

Answer 1

The correct option is A $820$

Given, $a_n=4n-1$ putting $n=1,2,3,......$
$a_1=3$
$a_2=7$
$a_3=11$
$a_4=15$
$\therefore$ Series is $3,7,11,15,......$
$S_{20}=\frac{20}{2}\{2\left(3\right)+(20-1)4\}$
$=10\{6+76\}$

$=10\times 82$

$=820$