Arithmetic Progressions
RD Sharma
CBSE
Grade 10
Question:
Which term of the A.P. $21, 42, 63, 84, \dots$ is $210$?
(a) $9^{\text{th}}$
(b) $10^{\text{th}}$
(c) $11^{\text{th}}$
(d) $12^{\text{th}}$
Step-by-Step Solution
Key Concept: $a = 21, d = 21$. $a_n = 21 + (n - 1)21 = 21n = 210 \Rightarrow n = 10$.
$21n = 210 \Rightarrow n = 10$. So $10^{\text{th}}$ term. [1.0 Mark]
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🎯 Official CBSE Marking Scheme:
Solving $n = 10$: 1.0 Mark
Correct Answer: $10^{\text{th}}$
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