OB*OC=cosa|OB||OC|
cosa=OB*OC/|OB||OC|

f=sinα･OA↑+sinβ･OB↑+sinγ･OC↑＝０↑
OAXf=sinbOAXOB+sinrOAXOC=sinbsinr|OA||OB|+sinrsin(-b)|OA||OC|
=sinbsinr|OA|(|OB|-|OC|)=0
OBxf=0
->f=0