Question

If the nth term of an AP is 7−3n then find the sum of first twenty five terms.

Answer 1

Answer:

An = 7-3n

A1 = 7-3(1)=4

A2= 7-3(2)=1

D = A2 + A1 = 1-4=-3

S25 = 25/2(2a + (n-1)D)

S25 = 25/2(2(4)+24(-3)

= 25/2(8-72)

= 25/2(-64)

= - 800