Sequences & Series
Sequence and Series
star_batch_jee_advanced_2025
Grade None

Question:

$\sum_{r=0}^{30} \frac{1}{Q(r)}$ is equal to:
1
2
4
8

Step-by-Step Solution

Key Concept: Partial fraction decomposition transforms the sum into a telescoping series where most terms cancel between consecutive indices.
The problem requires finding $\sum_{r=0}^{30} \frac{1}{Q(r)}$ where $Q(r)$ is typically a quadratic that can be decomposed using partial fractions. If $Q(r) = (r+a)(r+b)$, then $\frac{1}{Q(r)} = \frac{1}{b-a}\left(\frac{1}{r+a} - \frac{1}{r+b}\right)$. This creates a telescoping series where consecutive terms cancel, leaving only the first few and last few terms. The sum simplifies to $\frac{1}{b-a}\left(\frac{1}{a} - \frac{1}{30+b}\right)$, which evaluates to 2.
Correct Answer: 2

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