What is the sum of the first (13) terms of the AP (15,25,35,\ldots)?
Answer and explanation
Correct answer: 975
Here, the first term is \(a=15\), the common difference is \(d=25-15=10\), and \(n=13\). Thus, \(S_{13}=\frac{13}{2}[2(15)+(13-1)\times10]=\frac{13}{2}(150)=975\). Therefore, 975 is correct. \(1040\) results if \(n\) common differences are incorrectly used in place of \((n-1)\). Exam tip: Always check the \((n-1)\) term in the AP sum formula.
Frequently asked questions
What is the correct answer to this question?
975
Why is this the correct answer?
Here, the first term is \(a=15\), the common difference is \(d=25-15=10\), and \(n=13\). Thus, \(S_{13}=\frac{13}{2}[2(15)+(13-1)\times10]=\frac{13}{2}(150)=975\). Therefore, 975 is correct. \(1040\) results if \(n\) common differences are incorrectly used in place of \((n-1)\). Exam tip: Always check the \((n-1)\) term in the AP 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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