Question

The sum of the first 20 terms of an AP is 1280, and the common difference is 20. Then the fourth term of the AP will be __.

• A. -64
• B. -66
• C. -62
• D. -60

Answer 1

The correct option is B

-66

Given, the sum of first 20 terms is 1280.

The formula to find out the sum of n terms of an AP is

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

Given,

$d=20,S_n=1280,n=20$
$\Rightarrow S_{20}=\frac{20}{2}(2a+(20-1\left)20\right)=1280$
$\Rightarrow 10(2a+19(20\left)\right)=1280$
From the above equation we can calculate the value of $a$,
We get a = -126
fourth term = $a_4$
$a_4=a+(4-1)d$
$=-126+(4-1)20$
$=-126+60=-66$