The length of the segment of a vertical line from a point outside a line to this line is called the distance from the point to the line.

Set the Equation of the line L to be Ax+By+C=0, and the coordinates of the point P to be (Xo, Yo), then the distance from the point P to the line L is： d=│AXo+BYo+C│ / √（A²+B²）。