Question

Which term of AP :21 , 18 ,15 ....is zero?

Answer 1

The given is in A.P.
⇔ Arithmetic Progression.

first term in the given series is 21. Therfore a=21

a+d=18
⇒ 21+d =18

d = -3

nth term= a+(n-1)d
⇒0=a+(n-1)d
⇒0=21+[n-1](-3)
⇒n-1 = -21/-3
⇒n-1 = 7
⇒n =8.
Therefore the 8 th term is zero.