Q29 / 54 · JEE Advanced 2016 (Paper 1) · Mathematics

Let f :R→R, g :R→R and h :R→R be differentiable functions such that fx=x3+3x+2, gfx=x and hggx=x , for all x∈R. Then,
 

  • A
    g ′ 2=115
  • Correct answer
    B
    h ′ 1=666
  • Correct answer
    C
    h0=16
  • D
    hg3=36

Solution

If f ′ x=3x2+3
⇒gfx= x
⇒ g ′ fx=1 f ′ x
Also f0=2    (Put x=0)
So, g ′ 2=1 f ′ 0 
⇒ g′2=13
If h(ggx=x
h ′ ggx=1 g ′ gx. g ′ x
If, ggx=1  g(fx)=x⇒g-1x=f(x)
⇒g x= g-11⇒g x=f1=6
To find, g'6 use x=1 in
g'fx=1f'1 as  f1=6

To find, g'236, use x=6 as
f6=236
∴h'1=1g'6.g'(236)=116.1111⇒h'1=666
If ggx=0
∴gx=g-10 ⇒gx=f0

⇒ gx=2⇒x=g-12 

⇒  x=f2 ⇒  x=16
∴h0=16
gx=3  ⇒  x=g-13

⇒x=f3  ⇒  x=38
So hg3=38

Hence, the correct answer is options: 'B', 'C'.

More Differentiation Questions