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If (a=120), (d=-9), and (n=13), find (a_n).

Answer and explanation

Correct answer: 12

The nth term of an AP is \(a_n=a+(n-1)d\). Thus, \(a_{13}=120+(13-1)(-9)=120-108=12\). Therefore, 12 is correct. Getting 21 is a common error caused by using the negative common difference \(-9\) with the wrong sign. Exam tip: always use \(n-1\) and keep a negative \(d\) in brackets while calculating.

Related tags

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

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