Question

the nth term of an AP is given by Tn=7-3n. find the sum of 25th terms of the AP.

Answer 1

The n th term of an AP is 7-3n

So putting 1 as n we get:

nth term = 7-3*1

=7-3

=4

So the 1st term is 4.

The 2nd term is :

7-3*2

=7-6=1

Hence 2nd term is 1.

Common difference =1-4=-3

AP=4,1,.....................7-3n

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

=25/2*[8+(25-1)(-3)]

=25/2[8-72]

=25/2*-64

=25*-32

=-800

Hope it helps you.