If #vec a# and #vec b# are two equal vectors inclined at the angle #theta# then value of #sin (theta/2)=?#
So putting it all together:
Here is a reference for the vector that bisects the angle between two vectors .
The reference tells us that the vector,
Therefore, the dot-product between the bisector and the either vector must contain the cosine of
The following is the dot-product with
Divide both sides by
Use the identity,