Use dot product of normals: cosθ=∣n1⋅n2∣∣n1∣∣n2∣\cos \theta = \frac{|\mathbf{n}_1 \cdot \mathbf{n}_2|}{|\mathbf{n}_1| |\mathbf{n}_2|}cosθ=∣n1∣∣n2∣∣n1⋅n2∣.
