In Figure, if the coordinates of P are (a, b, c), then the coordinates of the feet of perpendiculars from P on XY, YZ and ZX-planes respectively, are

(a, b, 0), (0, b, c), (a, 0, c)

Clearly, PD, PE and PF are the perpendiculars from P on XY, YZ and ZX-planes respectively. So, D, E and F are the feet of the perpendiculars from P on XY, YZ and ZX-planes. The coordinates of D, E and F are D (a, b, 0), E(0, b, c) and F (a, 0, c) respectively.