What is the sum of the first (9) terms of the AP (11,22,33,\ldots)?
Answer and explanation
Correct answer: 495
Here, the first term is \(a=11\), the common difference is \(d=11\), and \(n=9\). The ninth term is \(a+(n-1)d=11+8\times11=99\). Hence, \(S_9=\frac{n}{2}(a+l)=\frac{9}{2}(11+99)=495\). Therefore, 495 is correct. An answer such as \(484\) can result from using an incorrect last term. Exam tip: verify the \(n\)th term before substituting values in the sum formula.
Frequently asked questions
What is the correct answer to this question?
495
Why is this the correct answer?
Here, the first term is \(a=11\), the common difference is \(d=11\), and \(n=9\). The ninth term is \(a+(n-1)d=11+8\times11=99\). Hence, \(S_9=\frac{n}{2}(a+l)=\frac{9}{2}(11+99)=495\). Therefore, 495 is correct. An answer such as \(484\) can result from using an incorrect last term. Exam tip: verify the \(n\)th term before substituting values in the sum formula.
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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