If S_y = 7381, what is the value of S_(y+2)?
Answer and explanation
Correct answer: 7626
We first identify y using S_y = y(y+1)/2. Since 121 × 122/2 = 121 × 61 = 7381, we have y = 121. To move from S_y to S_(y+2), add the next two natural-number terms, y+1 and y+2. Thus S_123 = S_121 + 122 + 123 = 7381 + 245 = 7626. Hence option C is correct. The answer is not found by adding 121 and 122, because those are not both new terms after S_121; the next terms are 122 and 123. The distractors reflect such indexing or addition errors.
Frequently asked questions
What is the correct answer to this question?
7626
Why is this the correct answer?
We first identify y using S_y = y(y+1)/2. Since 121 × 122/2 = 121 × 61 = 7381, we have y = 121. To move from S_y to S_(y+2), add the next two natural-number terms, y+1 and y+2. Thus S_123 = S_121 + 122 + 123 = 7381 + 245 = 7626. Hence option C is correct. The answer is not found by adding 121 and 122, because those are not both new terms after S_121; the next terms are 122 and 123. The distractors reflect such indexing or addition errors.
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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