Question

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

Answer 1

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

a=21
a+d=18
--------------
d = -3

⇒a(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.

Answer 2

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