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

If a line, y = mx + c is a tangent to the circle, (x – 3)2 + y2 = 1 and it is perpendicular to a line L1, where L1 is the tangent to the circle, x2 + y2 = 1 at the point $\left( {{1 \over {\sqrt 2 }},{1 \over {\sqrt 2 }}} \right)$, then :

  1. A c<sup>2</sup> + 6c + 7 = 0 Correct answer
  2. B c<sup>2</sup> - 7c + 6 = 0
  3. C c<sup>2</sup> – 6c + 7 = 0
  4. D c<sup>2</sup> + 7c + 6 = 0

Solution

For circle x<sup>2</sup> + y<sup>2</sup> = 1 tangnet at point P$\left( {{1 \over {\sqrt 2 }},{1 \over {\sqrt 2 }}} \right)$ is <br><br>T = 0 <br><br>$\Rightarrow$ ${1 \over {\sqrt 2 }}x + {1 \over {\sqrt 2 }}y - 1 = 0$ <br><br>$\Rightarrow$ x + y - $\sqrt 2$ = 0 <br><br>Perpendicular to the line is <br><br>x - y + c = 0 <br><br>This is tangent to the circle (x – 3)<sup>2</sup> + y<sup>2</sup> = 1 <br><br>Also we know, perpendicular distance from center = radius <br><br>$\therefore$ $\left| {{{3 - 0 + c} \over {\sqrt 2 }}} \right|$ = 1 <br><br>$\Rightarrow$ |3 + c| = ${\sqrt 2 }$ <br><br>$\Rightarrow$ ${\left( {3 + c} \right)^2} = 2$ <br><br>$\Rightarrow$ 9 + c<sup>2</sup> + 6c = 2 <br><br>$\Rightarrow$ c<sup>2</sup> + 6c + 7 = 0

About this question

Subject: Mathematics · Chapter: Circles · Topic: Equation of a Circle

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