Multiple choice

In the series $3, 9, 15$, .., what will be the ${ 21 }^{ st }$ term ?

  1. $177$
  2. $121$
  3. $123$
  4. $129$
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C Correct answer
Explanation

Arithmetic progression: a = 3, d = 6. 21st term = a + 20d = 3 + 20*6 = 123.

AI explanation

The sequence is an arithmetic progression with the first term a = 3 and a common difference d = 6. To find the 21st term, use the formula for the nth term of an arithmetic progression, which is a + (n - 1)d. Substituting the values gives 3 + (21 - 1)6, which simplifies to 3 + 120. The result is 123.

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