What is three times the sum of the first (18) natural numbers?
Answer and explanation
Correct answer: 513
The sum of the first \(n\) natural numbers is \(\frac{n(n+1)}{2}\). Thus, \(S_{18}=\frac{18\times19}{2}=171\). Three times this sum is \(3\times171=513\), so 513 is correct. A nearby value such as 503 results from not applying the sum formula correctly. Exam tip: first find the sum using \(\frac{n(n+1)}{2}\), then apply the required multiplication.
Frequently asked questions
What is the correct answer to this question?
513
Why is this the correct answer?
The sum of the first \(n\) natural numbers is \(\frac{n(n+1)}{2}\). Thus, \(S_{18}=\frac{18\times19}{2}=171\). Three times this sum is \(3\times171=513\), so 513 is correct. A nearby value such as 503 results from not applying the sum formula correctly. Exam tip: first find the sum using \(\frac{n(n+1)}{2}\), then apply the required multiplication.
Which subject and chapter does this question cover?
This is a Class 9 Mathematics question. Chapter: Sequences and Progressions. Topic: Sum of first n natural numbers.
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