If \(a_n=\frac{n(n+3)}{2}\), what is the value of \(a_9\)?
Answer and explanation
Correct answer: 54
Substitute \(n=9\) in \(a_n=\frac{n(n+3)}{2}\): \(a_9=\frac{9(9+3)}{2}=\frac{9\times12}{2}=54\). Therefore, 54 is correct. A value such as 48 may result from an error in evaluating \(9+3\) or in multiplication. Exam tip: after substituting \(n\), simplify the expression inside brackets first.
Frequently asked questions
What is the correct answer to this question?
54
Why is this the correct answer?
Substitute \(n=9\) in \(a_n=\frac{n(n+3)}{2}\): \(a_9=\frac{9(9+3)}{2}=\frac{9\times12}{2}=54\). Therefore, 54 is correct. A value such as 48 may result from an error in evaluating \(9+3\) or in multiplication. Exam tip: after substituting \(n\), simplify the expression inside brackets first.
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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