Maths-
General
Easy

Question

The function 2x3 – 9x2 + 12x – 3 is increasing when x belongs to the interval

  1. (-infinity, 2) È (2, infinity)    
  2. (-infinity, 1) È (2, infinity)    
  3. (-infinity, 3) È (3, infinity)    
  4. None    

hintHint:

We are given a function. We have to find the interval in which the function is increasing.

The correct answer is: (-infinity, 1) È (2, infinity)


    The given function is f left parenthesis x right parenthesis space equals 2 x cubed space minus space 9 x squared space plus space 12 x space minus space 3
    To find the extremum value, we will take the derivate of the function and equate it to zero
    f left parenthesis x right parenthesis space equals 2 x cubed space minus 9 x squared space plus space 12 x space minus space 3
T a k i n g space t h e space f i r s t space o r d e r space d e r i v a t i v e
f apostrophe left parenthesis x right parenthesis space equals space 6 x squared space minus space 18 x space plus space 12
f apostrophe left parenthesis x right parenthesis space equals space 6 left parenthesis x squared space minus space 3 x space plus space 2 right parenthesis
N o w comma space w e space w i l l space e q u a t e space i t space t o space z e r o.
space space space space space 0 equals space 6 left parenthesis x squared space minus 2 x space minus x space plus space 2 right parenthesis space space... left parenthesis b y space f a c t o r i s a t i o n right parenthesis
space space space space space 0 equals space 6 left parenthesis x minus 2 right parenthesis left parenthesis x minus 1 right parenthesis
space space space space R e a r r a n g i n g space w e space g e t space
6 left parenthesis x space minus space 2 right parenthesis left parenthesis x space minus space 1 right parenthesis space equals space 0
space space space space
S o comma space x space minus space 2 space equals space 0 space o r space space x space minus space 1 space equals space 0
space space space space space space space x space equals space 2 space space o r space space x space equals space 1
    Point 2 and 1 can be minima or maxima
    To find the interval, we will consider two cases
    1 points greater that 2
    2) points less than 1
    We will substitute any arbitrary value of the interval in first derivative and see if the function is increasing or decreasing.
    fIf first derivative is positive the given function is increasing and if first derivative is negative the given function is decreasing.
    Case 1) x > 2
    We will take x = 4
    f'(4) = 6(4 - 2)(4 - 1)
    = 36
    Hence, above 2 the value is increasing.
    Case 2) x< 1
    We will take x = 0
    f'(0) = 6(0 - 2)(0 - 1)
    = 12
    Hence, the below 0 the value of the function is increasing.
    So, the interval will be (-∞, 1)È(2,∞)

    We should know the way to find the minimum or maximum value.

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