Q49 / 75 · JEE Main 2020 (08 Jan Shift 1) · Mathematics

Let fx=sintan-1x+sincot-1x2-1, x>1. If dydx=12ddxsin-1fx and y3=π6, then y-3 is equal to:

  • A
    2π3
  • B
    -π6
  • Correct answer
    C
    5π6
  • D
    π3

Solution

Let tan-1x=θ

⇒x=tanθ ⇒sinθ=x1+x2

So, y=x1+x2+11+x22-1

⇒y=x+121+x2-1

⇒y=2x1+x2=fx

Now, dydx=1211-f2×f'x

=1211-4x21+x22fx

=1+x22x2-1f'x

=1+x22x2-1×21+x2-2x21+x22

⇒dydx=1-x2x2-11+x2

⇒dy=1-x2x2-11+x2dx

Integrating both sides with respect to x

∫dy=∫1-x2x2-11+x2dx

⇒y=-tan-1x+c

Given, y3=π6⇒π6=-π3+c⇒c=π2

y=-tan-1x+π2=cot-1x

Now, y-3=cot-1x-3=5π6

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

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