Question

whcih term of 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.


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Answer 2

a=21,d=-3,an=0, we need to find n
an=a+(n-1)*D
0=21+(n-1)*(-3)
-21=n-1*-3
-21/-3=(n-1)
7=n-1
n=8
so the 8th term is 0