Q19 / 46 · NTA Abhyas (JEE Main) 2020 · Mathematics · Differentiation

Let ϕ(x) be the inverse of the function f(x) and f′x=11+x5, then d dx ϕ x is equal to

  • A
    11+ϕx5
  • B
    11+fx5
  • Correct answer
    C
    1+ϕx5
  • D
    1+f(x)

Solution

We have, ϕx=f-1(x)

⇒x=f[ϕx]

On differentiating both sides w.r.t. x, we get

1=f'ϕx.ϕ'(x)

⇒ϕ'x=1f'ϕx ...(i)

Since, f′x=11+x5  (given)

f'ϕx=11+ϕx5

From Equation (i),

ϕ'x=1f'[ϕx]=1+ϕx5

Hence, the correct answer is option: 'C'.

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