Zero term of arithmetic progression

CBSE 2026 30/8/1 · MATH · Easy · Arithmetic Progressions · MCQ · 1 mark

Which term of A.P. $21, 18, 15, \dots$ is zero ?

Answer

First term $a = 21$ , common difference $d = 18 - 21 = -3$ .

$a_n = a + (n - 1)d = 0 \implies 21 + (n - 1)(-3) = 0 \implies 3(n - 1) = 21 \implies n - 1 = 7 \implies n = 8$

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