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If (a_1=12) and it is not (a_n=3a_{n-1}+2) but an AP with (a_n=12+(n-1)d), and (a_9=68), what is (a_{22})?

Answer and explanation

Correct answer: 159

Using the AP formula, \(a_9=12+(9-1)d\). Thus, \(68=12+8d\), so \(d=7\). Now \(a_{22}=12+(22-1)\times7=12+147=159\). Choosing 166 would result from using an incorrect common difference instead of 7. Exam tip: first find \(d\) from the given term, then use \(n-1\) for the required term.

Related tags

Arithmetic ProgressionNth TermCommon DifferenceClass 10 MathematicsAp FormulaFinding The $N$Th Term Of An ApFinding The N Th Term Of An ApArithmetic Progressions (Ap)

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