The condition for the lines $l x+m y+n=0$ and $l_1 x+m_1 y+n_1=0$ to be conjugate with respect to the circle $x^2+y^2=r^2$, is

• Correct answer
  A

  $r^2\left(I_1+m m_1\right)=n n_1$
• B

  $r^2\left(I_1-m m_1\right)=n n_1$
• C

  $r^2\left(I_1+m m_1\right)+n n_1=0$
• D

  $r^2\left(I m_1+I_1 m\right)=n n_1$

Solution

Condition for conjugate line is $$ r^2\left(I_1+m m_1\right)=n n_1 $$

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