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Maxima and Minima

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Estimated time: 13 minutes
CBSE: Class 12

Introduction

Maxima and minima are used to find the highest or lowest value of a quantity, such as greatest profit, shortest distance, maximum area, or minimum cost.

This topic forms an important part of Applications of Derivatives and connects graph interpretation, increasing-decreasing behavior, and optimization problems.

CBSE: Class 12

Definition: Maxima and Minima

A function may attain a maximum value at a point if its value there is greater than nearby values, and it may attain a minimum value if its value there is smaller than nearby values. These are often called extreme values.

CBSE: Class 12

Definition: Critical Point

A point in the domain of a function is called a critical point if either the derivative is zero there or the derivative does not exist there. Critical points are checked while locating possible maxima or minima.

CBSE: Class 12

Types of Extrema

Type Meaning
Local maximum Greatest value in a small neighborhood of a point
Local minimum Smallest value in a small neighborhood of a point
Absolute maximum Greatest value on the entire given interval
Absolute minimum Smallest value on the entire given interval
CBSE: Class 12

Theorem: First Derivative Test

Let c be a critical point of a continuous function f:

  • Local Maxima: f'(x) changes sign from positive to negative as x increases through c.

  • Local Minima: f'(x) changes sign from negative to positive as x increases through c.

  • Point of Inflection: f'(x) does not change sign as x passes through c (it is neither a maxima nor a minima).

CBSE: Class 12

Theorem: Second Derivative Test

Assume f'(c) = 0 and the second derivative exists at c:

  • Local Maxima: f''(c) < 0

  • Local Minima: f''(c) > 0

  • Test Fails: f''(c) = 0. If this happens, you must go back and use the First Derivative Test to check if it is a maxima, minima, or point of inflection.

CBSE: Class 12

Absolute Maxima and Minima

For a continuous function on a closed interval \([a,b]\), absolute maxima and minima are found by comparing function values at:

  • critical points inside the interval,

  • the left endpoint \(a\), and

  • the right endpoint \(b\).

This comparison is necessary because the largest or smallest value may occur at an endpoint.

CBSE: Class 12
Maharashtra State Board: Class 12

Key Points: Maxima and Minima

  • Maxima and minima are extreme values of a function.

  • Critical points occur where \(f'(x)=0\) or \(f'(x)\) is not defined.

  • If \(f'(x)\) changes from positive to negative, the function has a local maximum.

  • If \(f'(x)\) changes from negative to positive, the function has a local minimum.

  • If \(f''(c) < 0\), there is a local maximum at \(x=c\).

  • If \(f''(c) > 0\), there is a local minimum at \(x=c\).

  • For absolute extrema on \([a,b]\), compare values at critical points and endpoints.

  • Not every critical point gives a maximum or minimum.

  • The second derivative test is quick, but the first derivative test is often more reliable in detailed reasoning.

Shaalaa.com | Application of Derivatives part 21 (First Derivative Test)

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