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Arithmetic Progressions — Maths STD 10 — Question

CBSE BoardEnglish MediumSTD 10MathsArithmetic Progressions5 Marks
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.

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