Three-dimensional Geometry - Finding dihedral angles

General questions can be built system is to find dihedral angle and distance

Dihedral angle quantity product than the upper modulus product cosA = a * b / (|a| * |b |)

Calculate the value of cosA, and then according to the previous judgment

If it is an acute angle, and calculated cosA & gt; 0, then the required angle for the A

If it is acute, and calculated cosA & lt; 0, then the required angle is (Pai - A)

If it is acute, and calculated cosA & lt; 0, then the required angle is (Pai - A)

If it is obtuse, and calculated cosA<0, then the required angle is (Pai-A)

If it is an obtuse angle, and you get cosA>0, then the required angle is (Pai-A)

If it is an obtuse angle, and you get cosA<0, then the required angle is A