In an A.P. :
Given a = 2, d = 8, Sn = 90, find ‘n’ and an
Here, a = 2, d = 8, Sn = 90
We know that,
Sn = [2 x 2 + (n -1)d]
180 =n[2 x 2 (n - 1) x 8]
180 = n(8n - 4)
180 = 8n2 - 4n
8n2 - 4n - 180 = 0
2n2 - n - 45 = 0 [Dividing by 4]
2n(n - 5) (2n + 9) = 0
n - 5 = 0 or 2n + 9 = 0
n = 5 or n =
But number of terms cannot be (- ve)
Now, a5 = a+ 4d
= 2 + 4 + 8
Hence, n = 5
an = 34
The first term of an AP is 5, the last term is 45 and the sum is 400. Find the number of terms and the common difference.