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If \(a_n=\frac{n(n+1)}{2}\), what is the value of \(a_6\)?

Answer and explanation

Correct answer: 21

Given \(a_n=\frac{n(n+1)}{2}\). Substituting \(n=6\), \(a_6=\frac{6(6+1)}{2}=\frac{6\times7}{2}=21\). Therefore, 21 is correct. A value such as 18 can result from using an incorrect value for \(n+1\). Exam tip: substitute the term number carefully and evaluate the brackets before simplifying.

Related tags

Sequences And ProgressionsExplicit RuleSequence TermsSubstitutionClass 9 MathematicsExplicit Or General RuleMathematicsClass 9 Mcq

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