A workbook asks (1+2+3+\cdots+70). What is the answer?
Answer and explanation
Correct answer: 2485
The sum of the first \(n\) natural numbers is \(\frac{n(n+1)}{2}\). Here, \(n=70\), so \(1+2+\cdots+70=\frac{70\times71}{2}=35\times71=2485\). Therefore, option C is correct. A value such as 2475 can result from a small multiplication or division error. Exam tip: divide the even factor by 2 first to calculate quickly.
Frequently asked questions
What is the correct answer to this question?
2485
Why is this the correct answer?
The sum of the first \(n\) natural numbers is \(\frac{n(n+1)}{2}\). Here, \(n=70\), so \(1+2+\cdots+70=\frac{70\times71}{2}=35\times71=2485\). Therefore, option C is correct. A value such as 2475 can result from a small multiplication or division error. Exam tip: divide the even factor by 2 first to calculate quickly.
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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