Prove that: cos(A-B)=cos A cos B + sin A sin B by using vector properties let p and q are two vector |p|=|q|=1 just write out the scalar product of p bar and q bar twice we can write scalar product of p and q like below p bar. Q bar=|p||q| cos (A-B) =1.1.cos(A-B) =Cos (A-B) and p bar .q bar =cos A cos B +sin A sin B there for cos(A-B)=cos A cos B +sin A sin B