The document discusses the formula for calculating the perpendicular distance from a point to a line. It states that the perpendicular distance is the shortest distance. The formula is given as d = (Ax1 + By1 + C)/√(A2 + B2), where (x1, y1) are the coordinates of the point and Ax + By + C = 0 is the equation of the line. An example is worked through to find the equation of a circle given its tangent line and center point. It also discusses how the sign of (Ax1 + By1 + C) indicates which side of the line a point lies.