Engineering
Mathematics
Monotonicity
Question

Let f(x) = (1 – x)2 sin2x + x2  x R and let g(x) =  1x(2 ​(t1)t+1ℓnt)f(t)dt      x(1,) 
 

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Linked Question 1

Consider the statements :

P : There exists some x  R such that f(x) + 2x = 2 (1 + x2).

Q : There exist some x  R such that 2f(x) + 1 = 2x (1 + x).

Then

P is false and Q is true

both P and Q are false

both P and Q are true

P is true and Q is false

Solution

Let F(x) = f(x) + 2x – 2 – 2x2

F(x) = (1 – x)2 sin2x + x2 + 2x – 2 – 2x2

F(x) = (1 – x)2 sin2x – (1 – x)2 – 1

F(x) = – (1 – x)2 cos2x – 1 = – 1 [(1 – x)2cos2x + 1]

F(x) < 0 xR F (x) = 0 is not possible for any x  R.  So P is false

Let G(x) = 2f(x) + 1 – 2x – 2x2

G(x) = 2(1 – x)2 sin2x + 2x2 + 1 – 2x – 2x2

G(x) = 2 [1 – 2x + x2] sin2x + (1 – 2x)

G(x) = (2sin2x + 1) (1 – 2x) + 2x2 sin2x

Now   G (x) = 0            sin2x = 2x12(x1)2

As  2x12(x1)2 [– 1/2, ), so Q is true.

Hence P is false and Q is true.

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Linked Question 2

Which of the following is true?

g is decreasing on (1, ).

g is decreasing on (1, 2) and increasing on (2, ).

g is increasing on (1, 2) and decreasing on (2, ).

g is increasing on (1, ).

Solution

 g(x)=1x(2 ​(t1)t+1lnt)f(t)dt

 g'(x)=(2(x1)x+1lnx)f(x)

 g'(x)=[24(x+1)lnx]f(x)

Consider h(x) = 2 –  4x+1  – ln x

h'(x) =  4(x+1)21x

h'(x) =  4xx22x1x(x+1)2

h'(x) =  (x1)2x(x+1)2  < 0

  h(x) is a decreasing function  x (1, )

h(x) < h(1) = 0

  h(x) < 0 x (1, )

  g(x) < 0 as f(x) > 0  xR g(x) is decreasing in x   (1,  ). Ans.

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