Choose the correct (a_n) for the sequence (4,13,28,49,\ldots).
Answer and explanation
Correct answer: \(3n^2+1\)
For option B, \(a_n=3n^2+1\) gives \(a_1=3(1)^2+1=4\), \(a_2=3(2)^2+1=13\), \(a_3=28\), and \(a_4=49\). Hence, it is the correct general term. For example, option A gives \(a_2=11\), which does not match the second term, 13. Exam tip: test a proposed rule for at least the first two or three terms.
Frequently asked questions
What is the correct answer to this question?
\(3n^2+1\)
Why is this the correct answer?
For option B, \(a_n=3n^2+1\) gives \(a_1=3(1)^2+1=4\), \(a_2=3(2)^2+1=13\), \(a_3=28\), and \(a_4=49\). Hence, it is the correct general term. For example, option A gives \(a_2=11\), which does not match the second term, 13. Exam tip: test a proposed rule for at least the first two or three terms.
Which subject and chapter does this question cover?
This is a Class 9 Mathematics question. Chapter: Sequences and Progressions. Topic: Explicit or general rule.
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