Dihedral Angle

The Angle between two Planes. The dihedral angle between the planes

$\displaystyle A_1x+B_1y+C_1z+D_1$	$\textstyle =$	$\displaystyle 0$	(1)
$\displaystyle A_2x+B_2y+C_2z+D_2$	$\textstyle =$	$\displaystyle 0$	(2)

$\begin{displaymath} \cos\theta={A_1A_2+B_1B_2+C_1C_2\over\sqrt{{A_1}^2+{B_1}^2+{C_1}^2}\sqrt{{A_2}^2+{B_2}^2+{C_2}^2}}. \end{displaymath}$

(3)