Find the sum of the first 15 natural numbers.
Answer and explanation
Correct answer: 120
The governing concept is the sum of the first n natural numbers, which is S_n = n(n + 1)/2. Here n = 15 because the sequence is 1, 2, 3, ..., 15. Substituting gives S_15 = 15 × 16 / 2. First divide 16 by 2 to obtain 8, and then multiply 15 × 8 = 120. Therefore, option C is correct. Option A is too small, while options B and D result from an incorrect multiplication or an incorrect interpretation of the number of terms. The sequence has exactly 15 terms and ends at 15.
Frequently asked questions
What is the correct answer to this question?
120
Why is this the correct answer?
The governing concept is the sum of the first n natural numbers, which is S_n = n(n + 1)/2. Here n = 15 because the sequence is 1, 2, 3, ..., 15. Substituting gives S_15 = 15 × 16 / 2. First divide 16 by 2 to obtain 8, and then multiply 15 × 8 = 120. Therefore, option C is correct. Option A is too small, while options B and D result from an incorrect multiplication or an incorrect interpretation of the number of terms. The sequence has exactly 15 terms and ends at 15.
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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