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

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Solution

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

a =21
$a_{2}$ =18
$d = a_{2} - a_{1} = 18 - 21 = - 3$

⇒nth term, $a_{n} = a + (n - 1) d$

⇒ $a + (n - 1) d = 0$

⇒ $21 + [n - 1] (- 3) = 0$

⇒ $n - 1 = \frac{- 21}{- 3}$

⇒ $n - 1 = 7$

⇒ $n = 8.$

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