N
ur
= 0 But
AP
r
a
=
−
uuur
r
r Therefore, (
) N
0
r
−a
⋅
=
r
r
r
       … (1)
This is the vector equation of the plane Cartesian form
Let the given point A be (x1, y1, z1), P be (x, y, z)
and direction ratios of  N
ur
 are A, B and C