Hi

Here is the situation

nis a normal vector to the plane

For a plane ax+by+cz+d=0, we know that a normal vector has coordinates (a,b,c)

P is a point picked at random on the plane

H is the projection of A on the plane

The distance from A to the plane is AH

The dot product ofAPandnis

AP.n= (AH+HP).n=AH.n+HP.n=AH.nsinceHPandnare perpendicular

|AP.n| = |AH.n|=AH ||n|| since AH and n are parallel

AH = |AP.n| / ||n||