If u is the angle between vectors a and b, as shown in Fig. 135, it follows from
the definition of the vector product that
sin u ½a b½ ½a½ ½b½
This result is less convenient to use than the one used so far, namely,
cos u a b ½a½ ½b½
since the scalar product a / b is easier to evaluate than the vector product a/b.