Sigma Percentile
JEE Main 2019 (9 January)
LEVELJEE Main

Animated Solution for Mathematics - Limits, Continuity and Differentiability: For each , let be the greatest integer less than or equal to . Then is equal to

Select Answer:

Visualized Solution

Evaluating

  • We need to evaluate the limit:
  • Notice the direction: , meaning approaches from the left side.
  • This involves two special functions: the Greatest Integer Function and the Modulus Function .

Defining the Neighborhood

  • Since approaches from the left, is a small negative number.
  • We can restrict our focus to the interval .
  • In this specific interval, both and have fixed, predictable behaviors.

Behavior of as

  • The Greatest Integer Function returns the largest integer less than or equal to .
  • For any strictly between and , the integer just below it is .
  • Therefore, as , we can safely say .

Behavior of as

  • The Modulus Function gives the absolute value.
  • By definition, if , and if .
  • Since our , it is negative, so .

Substituting the Piecewise Values

  • Let's take our original expression:
  • Substitute and into it.
  • We get:

Algebraic Simplification

  • Look at the fraction:
  • Since but , we can cancel the common factor from the numerator and denominator.
  • This leaves us with:

Simplifying

  • We have a term.
  • Recall the odd function property of sine: .
  • Applying this, becomes .

Cleaning up the Expression

  • We have:
  • Divide by to get .
  • The expression simplifies beautifully to:

Final Limit Evaluation

  • Now the expression is continuous at .
  • We can directly substitute to evaluate the limit.
  • This gives our final answer: .

The Sigma Insight: Evaluation of Limits & L'Hopital's Rule

Solution Diagram

Analyzing the Setup

The problem asks us to evaluate the limit:
The notation defines our universe as the interval . In this specific neighborhood, the behavior of the piecewise functions becomes rigid and predictable.

Defining Local Behavior

First, consider the Greatest Integer Function, . By definition, it returns the largest integer less than or equal to . For any in the interval , the integer immediately to the left is . Thus, we have:
Next, consider the Modulus Function, . We know that for all . Since our interval is strictly negative, we substitute:

The Master Equation

Now, we substitute these defined values back into our original expression:
We observe a common factor of in both the numerator and the denominator. Because the limit process implies approaches but never equals , we are mathematically justified in canceling :

Final Calculation

Recall the odd function property of trigonometry, where . Therefore, . Substituting this into our expression yields:
Dividing by effectively flips the signs of the numerator, transforming into . The expression simplifies to:
Since the expression is now continuous at , we perform the final substitution:
The final result of the limit is .

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