Question

the sum of the first n terms of an Ap is 3n+n2. find first term (a) , common difference (d) ,fifteenth term, sum of first 25terms​

Answer 1

Step-by-step explanation:

Sn = 3n+ n2

Let n=1,2

So, s1= 3×1+1×1

=3+1

=4

So a= 4

S2= 3×2+2×2

=10

Now, t2= s2-s1

= 10-4=6

So, t1= 4 and t2 =6

Now,common difference (d) =t2-t1

= 6-4=2

Now, t15=a+14d

= 4+14×2=32

S25= 25/2(2×4+(25-1)2)

=25×32

= 800

Hope it helps........