Answer: Is perpendicular to line joining centres.
- A Passes through the origin in every possible configuration
- B Is perpendicular to line joining centres
- C Is parallel to the x-axis regardless of where the centres lie
- D Cannot be determined without knowing the radii of all three circles
Correct answer: B. Is perpendicular to line joining centres
Explanation: The radical axis of two circles is always perpendicular to the line joining their centres. This is a fundamental property.
The slope of a line is the ratio of vertical change (rise) to horizontal change (run) between any two points on it - equivalently, the tangent of the angle θ the line makes with the positive x-axis.