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Question #41338

Difficulty: 8/10 | 🟢 Free

The Question

Evaluate the sum S=∑n=1120([n+1n]−[n])S = \sum_{n=1}^{120} \left( \left[ \sqrt{n} + \frac{1}{\sqrt{n}} \right] - \left[ \sqrt{n} \right] \right) where [x][x] denotes the greatest integer less than or equal to xx.

Options

  • A.

    21

  • B.

    10

  • C.

    20

  • D.

    11

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Bridge

Practice problems that bridge the gap between foundational theory and advanced practical application.

Concepts Behind This Question

Step Function Discontinuities|Sum to n terms vs. General Term|Algebraic Simplification

Why This Question Is Tricky

Common Mistakes

This question tests the summation of a series involving step functions where the general term cannot be simplified using standard algebraic limits or telescoping methods.

The trap lies in assuming that the addition of a vanishing term 1/n1/\sqrt{n} to n\sqrt{n} only affects the greatest integer function at exactly the boundary points n=(k+1)2−1n = (k+1)^2 - 1, leading to an incorrect count of 1 jump per interval.

Students often fail to partition the domain into intervals [k2,(k+1)2−1][k^2, (k+1)^2 - 1] and rigorously solve the inequality n+1/n≥k+1\sqrt{n} + 1/\sqrt{n} \ge k+1.

By doing so, they miss that the function jumps for the last two integers of every interval for k≥2k \ge 2, and all three for k=1k=1, rendering 'General Term' approximations invalid.

Quick Facts About This Question

Question Information

SubjectMathematics
ChapterContinuity and Differentiability - Class 12
ConceptStep Function Discontinuities
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree