What is the twelfth term of the sequence (17, 19, 21, 23, ...)?
Answer and explanation
Correct answer: 39
The terms form an arithmetic sequence because each term increases by 2. Its first term is a₁ = 17 and its common difference is d = 2. The nth-term formula is aₙ = a₁ + (n − 1)d. For n = 12, a₁₂ = 17 + (12 − 1)2 = 17 + 22 = 39. Therefore option B is correct. Another way to see this is that moving from the first term to the twelfth term requires 11 equal jumps, each of size 2, so the total increase is 22. Option A corresponds to only 10 jumps, while options C and D are too large and would require differences greater than the stated pattern. The answer must be checked against the indexing: the first listed term is the first term, not the zeroth term.
Frequently asked questions
What is the correct answer to this question?
39
Why is this the correct answer?
The terms form an arithmetic sequence because each term increases by 2. Its first term is a₁ = 17 and its common difference is d = 2. The nth-term formula is aₙ = a₁ + (n − 1)d. For n = 12, a₁₂ = 17 + (12 − 1)2 = 17 + 22 = 39. Therefore option B is correct. Another way to see this is that moving from the first term to the twelfth term requires 11 equal jumps, each of size 2, so the total increase is 22. Option A corresponds to only 10 jumps, while options C and D are too large and would require differences greater than the stated pattern. The answer must be checked against the indexing: the first listed term is the first term, not the zeroth term.
Which subject and chapter does this question cover?
This is a Class 9 Mathematics question. Chapter: Sequences and Progressions. Topic: nth term.
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