The sum of the first (72) natural numbers is (2628). What is the sum of the first (71) natural numbers?
Answer and explanation
Correct answer: 2556
The sum of the first 72 natural numbers includes the last number, 72. Therefore, subtract 72 from 2628 to get the sum of the first 71 natural numbers: \(2628-72=2556\). Option B results from the common mistake of subtracting 71; the number removed is 72. Exam tip: to obtain \(S_{n-1}\) from \(S_n\), subtract \(n\).
Frequently asked questions
What is the correct answer to this question?
2556
Why is this the correct answer?
The sum of the first 72 natural numbers includes the last number, 72. Therefore, subtract 72 from 2628 to get the sum of the first 71 natural numbers: \(2628-72=2556\). Option B results from the common mistake of subtracting 71; the number removed is 72. Exam tip: to obtain \(S_{n-1}\) from \(S_n\), subtract \(n\).
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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