Q4 / 92 · NTA Abhyas (JEE Main) 2020 · Mathematics · Parabola

If two parabolas y2=4ax–k and x2=4ay–k have only one common point P, then the equation of normal to y2=4ax–k at P is

  • Correct answer
    A
    y+x=4a
  • B
    y+x=2a
  • C
    y+x=4
  • D
    y+x=2

Solution

Parabolas y2=4ax–k and x2=4ay–k touche each other at line y=x 
(∵ both parabola are inverse of each other)
⇒ y=x is a common tangent at P
⇒ we get point P by solving y2=4ax–k and y=x
⇒ point of contact P is 2a,2a
⇒ equation of normal at P2a,2a is x+y=4a

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

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