A teacher asked to add numbers from (1) to (15). What is the correct sum?
Answer and explanation
Correct answer: 120
The sum of the first 15 natural numbers is found using \(S_n=\frac{n(n+1)}{2}\). Here, \(n=15\), so \(S_{15}=\frac{15\times16}{2}=120\). Therefore, 120 is the correct answer. 125 may result from an arithmetic error, but applying the formula gives 120. Exam tip: For the sum from 1 to \(n\), use \(\frac{n(n+1)}{2}\).
Frequently asked questions
What is the correct answer to this question?
120
Why is this the correct answer?
The sum of the first 15 natural numbers is found using \(S_n=\frac{n(n+1)}{2}\). Here, \(n=15\), so \(S_{15}=\frac{15\times16}{2}=120\). Therefore, 120 is the correct answer. 125 may result from an arithmetic error, but applying the formula gives 120. Exam tip: For the sum from 1 to \(n\), use \(\frac{n(n+1)}{2}\).
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.
