What is the value of (S_{33}+S_{66}-S_{22})?
Answer and explanation
Correct answer: 2519
Here, \(S_n\) denotes the sum of the first \(n\) natural numbers, so \(S_n=\frac{n(n+1)}{2}\). Thus, \(S_{33}=\frac{33\times34}{2}=561\), \(S_{66}=\frac{66\times67}{2}=2211\), and \(S_{22}=\frac{22\times23}{2}=253\). Therefore, \(S_{33}+S_{66}-S_{22}=561+2211-253=2519\). An answer such as 2496 can result from calculating one of the sums or the subtraction incorrectly. Exam tip: evaluate each \(S_n\) separately before combining the results.
Frequently asked questions
What is the correct answer to this question?
2519
Why is this the correct answer?
Here, \(S_n\) denotes the sum of the first \(n\) natural numbers, so \(S_n=\frac{n(n+1)}{2}\). Thus, \(S_{33}=\frac{33\times34}{2}=561\), \(S_{66}=\frac{66\times67}{2}=2211\), and \(S_{22}=\frac{22\times23}{2}=253\). Therefore, \(S_{33}+S_{66}-S_{22}=561+2211-253=2519\). An answer such as 2496 can result from calculating one of the sums or the subtraction incorrectly. Exam tip: evaluate each \(S_n\) separately before combining the results.
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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