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.