Question

In an A.P, the sum of three middle most terms is 201 and the sum of the last three terms is 363. If there are 39 terms in the given A.P, then the correct option(s) is/are ​

Answer 1

Step-by-step explanation:

First term (a)=1

Last term (tn)=20

Sum of terms (Sn)=399

We know that,

tn=a+(n−1)d

Sn=2n(2a+(n−1)d)

Sn=2n(a+(a+(n−1)d))

⇒Sn=2n(a+tn)

⇒399=2n(1+20)

⇒399=221n

⇒21n=399×2

⇒n=21798

⇒n=38