the rule is for the distance between two parallel planes(Ax+By+Cz=D

_{1}and Ax+By+Cz=D

_{2}) in 3D space and can be written as the following:

|D

_{1}-D

_{2}|

------------ =d

_{1-2}

|Ai+Bj+Ck|

where (A,B,C) is the normal vector for the parallel planes, i, j andk are unit vectors and d

_{1-2}is the distance between the planes

so.. I know that |Ai+Bj+Ck| is the length of the normal vector and

|D

_{1}-D

_{2}| has something to do with the distance between them in a certain direction.

the usual method I use to do this is

Ax+By+Cz+D

------------ =d

_{1-2}

|Ai+Bj+Ck|

where x,y,z are the coordinates of a point I choose

so please can anyone help me prove this rule?