prove these identities

[icelated]

I have 3 problems in my supplement package that has no solutions and i cannot possibly figure out how to do them.
They are all basically the same and if i knew how to do the first one i could get the rest

I dont even know where to begin.

cos(m)cos(n) = 1/2(cos( m + n ) + cos( m - n )

I have no idea on what to do

 

[Deleted member 4993]

I have 3 problems in my supplement package that has no solutions and i cannot possibly figure out how to do them.
They are all basically the same and if i knew how to do the first one i could get the rest

I dont even know where to begin.

cos(m)cos(n) = 1/2(cos( m + n ) + cos( m - n )

I have no idea on what to do

Use:

cos(A+B) = cos(A)*cos(B) - sin(A)*sin(B)

and

cos(A-B) = cos(A)*cos(B) + sin(A)*sin(B)