Sequences & Series
Sequence and Series
Allen Star Batch
Grade 11

Question:

$\sum_{r=1}^{99} r(r^2 + r + 1)$ is equal to:
$102-100!$
$100(100!)-1$
$99(100!)-1$
$100(99!)-1$

Step-by-Step Solution

Key Concept: Recognizing that the expression telescopes when rewritten as a difference of consecutive factorial-like terms.
The sum $\sum_{r=1}^{99} r[(r+1)^2 - r] = \sum_{r=1}^{99} r[r^2 + 2r + 1 - r] = \sum_{r=1}^{99} r(r^2 + r + 1) = \sum_{r=1}^{99} [(r+1)(r+1)! - r!] $ telescopes. This evaluates to $100! - 1 = 100(100!) - 1$.
Correct Answer: 2

Master Sequences & Series with Mathbee

Practice this topic under real exam conditions with strict timers, or ask our AI Mentor to explain the concepts step-by-step.

Start Practicing for Free