add volume of parallelapiped

uni/mmme/1026_maths_for_engineering/vectors.md

@@ -121,7 +121,7 @@ vector such that the three vectors from a right handed system:
     This is technically not a determinant because not all the elements are numbers but shhhhhh...
 
 
-### Scalar Triple Product
+### Scalar Triple Product (and co-planar vectors)
 
 \begin{align*}
         [ \pmb a, \pmb b, \pmb c ] &= \pmb a \cdot (\pmb b \times \pmb c) \\
@@ -211,6 +211,10 @@ $$\cos\theta = \frac{\pmb a \cdot \pmb b}{|\pmb a||\pmb b|} = \frac{a_1b_1 + a_2
 
 $$area = |\pmb a||\pmb b|\sin\theta = |\pmb a \times \pmb b|$$
 
+### Volume of a Parallelepiped
+
+$$volume = | [ \pmb a, \pmb, b, \pmb c ] |$$
+
 ### Equation of a Straight Line
 
 A straight line can be specified by