Question

The nth term of an AP is 7-4n. Find the sum of first 5 terms.

Answer 1

So,

a n= 7- 4n

We need,

d = ? and a=?

so,

a₁= 7 - 4x1 = 3

a₂= 7-4x2= -1

d= a₂-a₁= -1 -3 = -4

Sum of first 5 terms in ap,

s n = n/2[2a+ (n-1)d]

s₅= 5/2[ 2x3 + 4x-4]

= 5/2[6-16]

=5/2x-10

= -25

Therefore,  

S₅= -25