Equation of lines in 2D is given as: Ax + By + C = 0
vector [A,B] is the line joining the origin to the point (A,B) is perpendicular to the 
line Ax + By + C = 0.

For 2 lines it would be
A1x + B1y + C1 = 0
A2x + B2y + C2 = 0

angle between two vectors u=[a,b] and v=[x,y]
	cos θ = (ax + by) / |u|*|v|

hence the angle between the perpendiculars is
	cos θ = (A1A2 + B1B2) / ( sqrt( A12 + B12) * sqrt(A22 + B22) )

this is the same as the angle between the two lines

similarly for 3D the equation of a plane is given by: Ax + By + Cz + D = 0
the angle between two such planes would be given by
	cos θ = (A1A2 + B1B2 + C1C2) / ( sqrt(A12 + B12 + C12) * sqrt(A22 + B22 + C22 ) )