The perpendicular distance from point P(x_{1},y_{1},z_{1}) to plane ax+by+cz+d = 0 is: D = |ax_{1}+by_{1}+cz_{1}+d|/sqrt($a^{2}$+$b^{2}$+$c^{2}$). This is one of the most frequently used formulas in JEE 3D geometry. To find the foot of the perpendicular, write the line through P with direction (a,b,c) (parallel to the normal), and find where it meets the plane.