Maths-
General
Easy

Question

The value of 'c' in Lagrange's mean value theorem for f space left parenthesis x right parenthesis equals x space left parenthesis x minus 2 right parenthesis squared in [0, 2] is

  1. 0    
  2. 1    
  3. 2/3    
  4. 3/2    

hintHint:

Lagrange mean value theorem states that for any two points on the curve there exists a point on the curve such that the tangent drawn at this point is parallel to the secant through the two points on the curve.

The correct answer is: 2/3


     Given : f space left parenthesis x right parenthesis equals x space left parenthesis x minus 2 right parenthesis squared is a polynomial
    We know that polynomials are continuous and differentiable in their range
    rightwards double arrow space f space left parenthesis x right parenthesis equals x space left parenthesis x minus 2 right parenthesis squared space i s space c o n t i n u o u s space a n d space d i f f e r e n t i a b l e space i n space t h e space r a n g e space left square bracket space 0 comma space 2 space right square bracket
    Simplify f(x) for ease of calculation
    rightwards double arrow space f space left parenthesis x right parenthesis equals x space left parenthesis x minus 2 right parenthesis squared space

rightwards double arrow space f space left parenthesis x right parenthesis equals x space left parenthesis x squared plus 4 space minus 4 x right parenthesis

rightwards double arrow space f space left parenthesis x right parenthesis equals x cubed plus 4 x space minus 4 x squared space....... space left parenthesis 1 right parenthesis

N o w comma space s u b s t i t u t e space t h e space v a l u e s space o f space x space a s space 0 space a n d space 2

rightwards double arrow space f left parenthesis 0 right parenthesis space equals space 0
rightwards double arrow space f left parenthesis 2 right parenthesis space equals space space 0 space

B y space L a g r a n g e s space M e a n space V a l u e space T h e o r e m

f apostrophe left parenthesis c right parenthesis space equals space fraction numerator f left parenthesis b right parenthesis space minus space f left parenthesis a right parenthesis over denominator b minus a end fraction

f apostrophe left parenthesis c right parenthesis space equals space fraction numerator f left parenthesis 2 right parenthesis space minus space f left parenthesis 0 right parenthesis over denominator 2 minus 0 end fraction space equals space 0 space space........ space left parenthesis 2 right parenthesis

N o w space d i f f e r e n t i a t i n g space left parenthesis 1 right parenthesis space

f apostrophe space left parenthesis x right parenthesis equals 3 x squared plus 4 space minus 8 x

f apostrophe left parenthesis c right parenthesis space equals space 3 c squared plus 4 space minus 8 c

F r o m space left parenthesis 2 right parenthesis

3 c squared plus 4 space minus 8 c space equals space 0

N o w space b y space u sin g space q u a d r a t i c space f o r m u l a comma space w e space f i n d space t h e space r o o t s space o f space c

c space equals space fraction numerator negative left parenthesis negative 8 right parenthesis space plus-or-minus square root of left parenthesis negative 8 right parenthesis squared space minus space 4 space cross times 3 cross times 4 end root over denominator 2 cross times 3 end fraction space equals space fraction numerator 8 space plus-or-minus space 4 over denominator 6 end fraction space equals space 2 space o r space 2 over 3

T h u s space c space equals space 2 over 3

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