If (S_n-S_{n-4}=250), what will be the value of (n)?
Answer and explanation
Correct answer: 64
Here, \(S_n-S_{n-4}\) represents the sum of the four natural numbers from the \((n-3)\)th term to the \(n\)th term: \(S_n-S_{n-4}=(n-3)+(n-2)+(n-1)+n=4n-6\). Hence, \(4n-6=250\), so \(4n=256\) and \(n=64\). Therefore, 64 is the correct option. For instance, if \(n=63\), the sum is \(60+61+62+63=246\), not 250. Exam tip: \(S_n-S_{n-k}\) always gives the sum of the last \(k\) terms.
Frequently asked questions
What is the correct answer to this question?
64
Why is this the correct answer?
Here, \(S_n-S_{n-4}\) represents the sum of the four natural numbers from the \((n-3)\)th term to the \(n\)th term: \(S_n-S_{n-4}=(n-3)+(n-2)+(n-1)+n=4n-6\). Hence, \(4n-6=250\), so \(4n=256\) and \(n=64\). Therefore, 64 is the correct option. For instance, if \(n=63\), the sum is \(60+61+62+63=246\), not 250. Exam tip: \(S_n-S_{n-k}\) always gives the sum of the last \(k\) terms.
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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