Question

find the radius of circle having the lines 3x+4y-1 and 6x+8y-12 has tangents

Answer 1

Since the slope of both the line are equal means these both lines are parallel tangent to the circle . This is possible if and only if there tangent to the diameter . Hence the distance between them will be the diameter and half of it will be the radius.
D=1 and r =0.5