Question

The locus of a point such that difference of the squares of the tangents from it to two given circles is constant, is given by

• A. A circle
• B. A line perpendicular to radical axis
• C. A line parallel to radical axis
• D. A pair of straight lines

Answer 1

Answer: (C)

A line parallel to radical axis

So, $PA-PB=C$
$(\sqrt{s_1}{)}^2-(\sqrt{s_2}{)}^2=c$
$s_1-s_2=c$
as we know, $s_1-s_2=0$ is a equation of radical axis.
So, then $s_1-s_2=c$ is a equation of line parallel to
radical axis.