cos(a+b)=cosacosb-sinasinb
sin(a+b)=sinacosb+cosasinb
tan(a+b)=(tana+tanb)/(1-tanatanb)
tan(a-b)=(tana-tanb)/(1+tanatanb)
sin(a-b)=sinacosb-cosasinb
cos(a-b)=cosacosb+sinasinb