Medium MCQ +4 / -1 PYQ · JEE Mains 2020

If the common tangent to the parabolas,
y2 = 4x and x2 = 4y also touches the circle, x2 + y2 = c2,
then c is equal to :

  1. A ${1 \over {\sqrt 2 }}$ Correct answer
  2. B ${1 \over {2\sqrt 2 }}$
  3. C ${1 \over 2}$
  4. D ${1 \over 4}$

Solution

$y = mx + {1 \over m}$ (tangent at y<sup>2</sup> = 4x) <br><br>y = mx – m<sup>2</sup> (tangent at x<sup>2</sup> = 4y) <br><br>${1 \over m} = - {m^2}$ (for common tangent) <br><br>m<sup>3</sup> = – 1 <br><br>$\Rightarrow$ m = - 1 <br><br>$\therefore$ Equation of tangent <br><br>y = –x –1 <br><br>x + y + 1 = 0 <br><br>This line touches circle whose center at (0, 0), <br><br>$\therefore$ apply p( Distance from center of the circle of the line ) = r ( Radius of the circle ) <br><br>c = $\left| {{{0 + 0 + 1} \over {\sqrt 2 }}} \right| = {1 \over {\sqrt 2 }}$

About this question

Subject: Mathematics · Chapter: Conic Sections · Topic: Parabola

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