What is the difference between the sums of the first (80) and first (75) natural numbers?
Answer and explanation
Correct answer: 390
When the sum of the first 75 natural numbers is subtracted from the sum of the first 80 natural numbers, only the terms from 76 to 80 remain. Thus, the difference is \(76+77+78+79+80=390\). Therefore, option A is correct. Although 395 is close, it is not the sum of these five numbers. Exam tip: In differences of partial sums, cancel the common initial terms and add only the remaining terms.
Frequently asked questions
What is the correct answer to this question?
390
Why is this the correct answer?
When the sum of the first 75 natural numbers is subtracted from the sum of the first 80 natural numbers, only the terms from 76 to 80 remain. Thus, the difference is \(76+77+78+79+80=390\). Therefore, option A is correct. Although 395 is close, it is not the sum of these five numbers. Exam tip: In differences of partial sums, cancel the common initial terms and add only the remaining 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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