Q31 / 90 · JEE Main 2018 (16 Apr Online) · Mathematics

If x=2cosec-1t and  y=2sec-1t, t≥1, then dydx is equal to

  • A
    yx
  • Correct answer
    B
    -yx
  • C
    -xy
  • D
    xy

Solution

Given, x=2cosec-1t=2cosec-1t2 and  y=2sec-1t=2sec-1t2, then we have

x·y=2sec−1t+cosec−1t2

And, we know that sec-1x+cosec-1x=π2,

⇒xy=2π4

On differentiating, both sides with respect to x, and applying product rule, we get

xdydx+y=0

⇒dydx=-yx.

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

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