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  • Which term of the AP: 21, 42, 63, 84... is 210? (A) 9th (B) 10th (C) 11th (D) 12th

Which term of the AP: 21, 42, 63, 84... is 210?

(A) 9th

(B) 10th

(C) 11th

(D) 12th

Answers (1)

Here,

a_n = 210 \\ \text{AP is } 21, 42, 63, 84, \ldots \\ \text{Here } a_1 = 21 \\ d = a_2 - a_1 \quad (\text{where } d \text{ is the common difference}) \\ d = 42 - 21 \quad (a_1 \text{ is the first term}) \\ d = 21 \\ a_n = 210 \\ a_1 + (n - 1) \cdot d = 210 \\ 21 + (n - 1) \cdot 21 = 210 \\ 21 + 21n - 21 = 210 \\ 21n = 210 \\
n = \frac{210}{21} \\ n = 10 \\ \text{Hence the } 10^{th} \text{ term of the AP is } 210

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infoexpert21

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