Solution:
Given AP is -6, -11/2, -5…
Here a = –6, d = (–11/2) + 6 = 1/2
Sum of n terms of AP, Sn = – 25
Using sum of n term of AP formula
⇒ n2 – 25n + 100 = 0
⇒ n2 – 20n – 5n + 100 = 0
⇒ n(n – 20) – 5(n – 20) = 0
⇒ (n – 20)(n – 5) = 0
⇒ (n – 20) = 0 or (n – 5) = 0
∴ n = 20 or n = 5
