Sunday, December 7, 2008

Intersection of a straight line and a circle

Equation of the circle: x² + y² = a²

Equation of the line: y = mx+c

A line does not intersect a circle if the length of the perpendicular to the line from the centre of the circle is greater than the radius of the circle.
|c/√(1+m²)|>a

A line intersects a circle if the length of the perpendicular to the line from the centre of the circle is less than the radius of the circle.

|c/√(1+m²)|
A line touches a circle if the length of the perpendicular to the line from the centre of the circle is equal to the radius of the circle.

|c/√(1+m²)| = a

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