Question
Which term of the AP $21, 18, 15, ....$ is zero?
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Answer
In the given AP, first term, $a = 21$ and common difference, $d = (18 - 21) = -3$
Let's its $n^{th}$ term be $0$.
Then $T_n = 0$
$\Rightarrow a + (n - 1)d = 0$
$\Rightarrow 21 + (n - 1) \times (-3) = 0$
$\Rightarrow 24 - 3n = 0$
$\Rightarrow 3n = 24$
$\Rightarrow n = 8$
Hence, the $8^{th}$ term of the given AP is $0$.
Let's its $n^{th}$ term be $0$.
Then $T_n = 0$
$\Rightarrow a + (n - 1)d = 0$
$\Rightarrow 21 + (n - 1) \times (-3) = 0$
$\Rightarrow 24 - 3n = 0$
$\Rightarrow 3n = 24$
$\Rightarrow n = 8$
Hence, the $8^{th}$ term of the given AP is $0$.
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