Sigma Percentile
JEE Main 2025 April
LEVELJEE Main

Animated Solution for Mathematics - Differentiation: If the function , where , attains its local maximum and local minimum values at and , respectively, such that , then is equal to:

Select Answer:

Visualized Solution

Introduction to

  • Given function:
  • Condition:
  • Local maximum at
  • Local minimum at

Condition for Extrema

  • At local extrema, the tangent is horizontal.
  • Therefore, the first derivative must be zero:

Finding

  • Differentiating with respect to :

Critical Points Equation

  • Set the derivative to zero to find critical points:
  • Divide the entire equation by :

Solving for

  • Factorize the quadratic equation:

Roots of

  • The critical points are and .
  • Since it is given that :
  • We can conclude that .

Identifying and

  • For a cubic with a positive leading coefficient ():
  • The first critical point is the local maximum:
  • The second critical point is the local minimum:

Using

  • Apply the given constraint:
  • Substitute and :

Finding the value of

  • Solve the equation for :
  • Since , we reject and get .

Substituting into

  • Substitute back into the original function:

Evaluating

  • We need to find the value of .
  • Substitute into the function:

Final Arithmetic

  • Perform the final arithmetic:

The Sigma Insight: Maxima and Minima

Solution Diagram

The Geometry of the Cubic

Imagine you are standing on the graph of the function . Because the leading coefficient is positive, this is a dynamic, sweeping path that climbs from negative infinity, reaches a peak, descends into a valley, and then surges upward again.
The points where this curve turns—the peak and the valley—are our local maximum and local minimum. We are told the maximum occurs at and the minimum at , with the constraint that .
Let us embark on the journey to find .

The Calculus of Motion

To find these turning points, we need to identify where the slope of the tangent line is perfectly horizontal. We calculate the first derivative, :
Applying the power rule, we obtain:
This derivative acts as our compass. To find the critical points, we set :
Dividing by simplifies our task immensely:

The Algebra of Extrema

Next, we factorize this quadratic equation. We look for two numbers that multiply to and add to , which are and . Thus, the equation becomes:
This yields two critical points: and . Since we are given that , we know for certain that .
For a cubic with a positive leading coefficient, the first critical point from the left is the local maximum, and the second is the local minimum. Therefore, and .

The Constraint and the Final Calculation

We are armed with the condition . Substituting our values, we get:
Rearranging this, we have , which factors to . Since the problem explicitly states , we must reject .
Thus, . With determined, our function is fully revealed:
Finally, we evaluate :
The journey from the abstract geometry of a cubic curve to the concrete value of is complete.

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