cos(a-b)-cos(a+b)=cosacosb+sinasinb-cosacosb+sinasinb=2sinasinb
cos(a-b)=cosacosb+sinasinb, cos(a+b)=cosacosb-sinasinb.
cosacosb+sinasinb-cosacosb+sinasinb= 2sinasinb
cos(a-b)-cos(a+b)=cosacosb+sinasinb-cosacosb+sinasinb=2sinasinb
cos(a-b)=cosacosb+sinasinb, cos(a+b)=cosacosb-sinasinb.
cosacosb+sinasinb-cosacosb+sinasinb= 2sinasinb