Question

prove cos(a+b)=cosacosb-sinasinb

Answer 1

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