Question

find the sum of first 25 terms of an AP whose nth term is 1-4n

Answer 1

Answer:

-1275

Step-by-step explanation:

nth term=1-4n

1st term=1-4*1

            =-3

25th term=1-4*25

               =1-100

                 =-99

sum of 25 terms=25/2{-3+(-99)}

                          =25/2{-102}

                           =25*-51

                             =-1275

Answer 2

Answer:

−1275

Step-by-step explanation:

nth term of AP =  $a_n=1-4n$

Substitute n  =1

$a_1=1-4(1)=-3$

Substitute n  =2

$a_2=1-4(2)=-7$

Substitute n  = 3

$a_3=1-4(3)=-11$

So, first term = a = -3

Common difference = d = -7-(-3)=-11-(-7)=-4

Formula of sum of first n terms = $S_n=\frac{n}{2}(2a+(n-1)d)$

Substitute n = 25

$S_{25}=\frac{25}{2}(2(-3)+(25-1)(-4))$

$S_{25}=−1275$

Hence the sum of first 25 terms is −1275