Answer :

Here first term = 21

Common difference = 18 - 21 = - 3


Let an be the term which is zero.


an = 0


a + (n - 1)d = 0


21 + (n - 1)(-3) = 0


21 - 3n + 3 = 0


3n = 24


n = 8


8th term of the given AP will be zero.


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