[Math] How to find radius of a circle when equations of parallel tangents are given

algebra-precalculuscirclessystems of equations

Equations of two parallel tangents to a circle are
$2x-4y-9=0$ and $6x-12y+7=0$. How do I find the radius of the circle?

I have tried many methods, but still I could not find the radius.
I know that the distance between two parallel tangents is equal to the diameter, but none of the contact point are given.

If I assume the centre to be $(a,b)$ and radius to be $r$ then when I apply the perpendicular distance of a line from a point I get two equations but I have three unknowns, so I cannot solve….any help will be appreciated

Best Answer

Hint: The distance between two parallel lines, $y=mx + c_1$ and $y=mx+c_2$ is $$d = \frac{|c_1-c_2|}{\sqrt{1+m^2}}.$$