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If (a_n=12n+q) and (a_{8n}-a_{3n}=780), what is the value of (n)?

Answer and explanation

Correct answer: 13

Given \(a_n=12n+q\), we get \(a_{8n}=12(8n)+q=96n+q\) and \(a_{3n}=12(3n)+q=36n+q\). Hence, \(a_{8n}-a_{3n}=(96n+q)-(36n+q)=60n\). Since \(60n=780\), \(n=13\). The constant \(q\) cancels because it occurs in both terms. Exam tip: substitute the required indices into the term formula first, then take their difference.

Related tags

Arithmetic ProgressionNth TermAlgebraic SubstitutionSequence IndicesLinear EquationsFinding The $N$Th Term Of An ApFinding The N Th Term Of An ApArithmetic Progressions (Ap)

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