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A sequence has (n)th term \(a_n=\frac{n(n+1)}{2}\). What is \(a_{12}\)?
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Answer and explanation
Correct answer: 78
Given \(a_n=\frac{n(n+1)}{2}\). Substituting \(n=12\), \(a_{12}=\frac{12(12+1)}{2}=\frac{12\times13}{2}=78\). The value 72 may result from incorrectly using 12 in place of \(n+1\). Exam tip: substitute the given value of \(n\) carefully before simplifying the formula.
Frequently asked questions
What is the correct answer to this question?
78
Why is this the correct answer?
Given \(a_n=\frac{n(n+1)}{2}\). Substituting \(n=12\), \(a_{12}=\frac{12(12+1)}{2}=\frac{12\times13}{2}=78\). The value 72 may result from incorrectly using 12 in place of \(n+1\). Exam tip: substitute the given value of \(n\) carefully before simplifying the formula.
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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