The document discusses the perpendicular distance formula. It states that the shortest distance from a point to a line is the perpendicular distance. It provides the formula for finding the perpendicular distance as d=|Ax+By+C|/√(A^2 +B^2) where Ax+By+C=0 is the equation of the line. It gives an example of finding the equation of a circle given the equation of a tangent line and the center point.