Question

the sum of first n terms of an AP is given by Sn=3n²/2+5n/2. find the 25th term of the AP

Answer 1

Sn=3n²/2+5n/2

Putting 1 as n we get:-

S1=3/2+5/2

=5+3/2

=8/2

=4

so a = 4.

Now S2=a+a+d

S2-S1=2a+d-a

=a+d

=2nd term.

Then putting n as 2 in the given question we get:

3*2²/2+5*2/2

=6+5

=11

S2=11

S2-S1=11-4=7

2nd term=7

a+d=7

4+d=7

d=7-4=3

So 25 th term

=a+(25-1)d

=a+24d

=4+24*3

=4+72

=76

Hope it helps you.