Find the sum of the first (20) natural numbers.
Answer and explanation
Correct answer: 210
The sum of the first \(n\) natural numbers is \(S_n=\frac{n(n+1)}{2}\). Therefore, \(S_{20}=\frac{20\times21}{2}=10\times21=210\). The value \(200\) may result from using \(20\times10\), but the correct formula must include \(n+1=21\). Exam tip: Natural numbers start from \(1\), so remember to use both \(n\) and \(n+1\) in the formula.
Frequently asked questions
What is the correct answer to this question?
210
Why is this the correct answer?
The sum of the first \(n\) natural numbers is \(S_n=\frac{n(n+1)}{2}\). Therefore, \(S_{20}=\frac{20\times21}{2}=10\times21=210\). The value \(200\) may result from using \(20\times10\), but the correct formula must include \(n+1=21\). Exam tip: Natural numbers start from \(1\), so remember to use both \(n\) and \(n+1\) in the formula.
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.
