What is the sum of the first (12) terms of the AP (9,18,27,\ldots)?
Answer and explanation
Correct answer: 702
Here, the first term is \(a=9\), the common difference is \(d=9\), and \(n=12\). The 12th term is \(a_{12}=a+(n-1)d=9+11\times9=108\). Therefore, \(S_{12}=\frac{12}{2}(9+108)=6\times117=702\). Hence, 702 is correct. Exam tip: First find the \(n\)th term, then use \(S_n=\frac{n}{2}(a+l)\).
Frequently asked questions
What is the correct answer to this question?
702
Why is this the correct answer?
Here, the first term is \(a=9\), the common difference is \(d=9\), and \(n=12\). The 12th term is \(a_{12}=a+(n-1)d=9+11\times9=108\). Therefore, \(S_{12}=\frac{12}{2}(9+108)=6\times117=702\). Hence, 702 is correct. Exam tip: First find the \(n\)th term, then use \(S_n=\frac{n}{2}(a+l)\).
Which subject and chapter does this question cover?
This is a Class 10 Mathematics question. Chapter: Arithmetic Progressions (AP). Topic: Finding the sum of the first $n$ terms of an AP.
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