Which number should be added to (S_{68}) to get (S_{75})?
Advertisement
Answer and explanation
Correct answer: 504
Since \(S_{75}=S_{68}+(69+70+\cdots+75)\), the required number is the sum of the terms from 69 to 75. There are 7 terms, so their sum is \(\frac{7}{2}(69+75)=\frac{7}{2}\times144=504\). Hence, 504 is correct. Exam tip: When finding \(S_b-S_a\), add the terms from \(a+1\) to \(b\).
Frequently asked questions
What is the correct answer to this question?
504
Why is this the correct answer?
Since \(S_{75}=S_{68}+(69+70+\cdots+75)\), the required number is the sum of the terms from 69 to 75. There are 7 terms, so their sum is \(\frac{7}{2}(69+75)=\frac{7}{2}\times144=504\). Hence, 504 is correct. Exam tip: When finding \(S_b-S_a\), add the terms from \(a+1\) to \(b\).
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.
Advertisement