Question

How do you use the sum to product formulas to write the sum or difference `cos(phi+2pi)+cosphi` as a product?

Answer 1

`rarrcos(phi+2pi)+cosphi=2cosphi`

Explanation:

`rarrcos(phi+2pi)+cosphi`

`=2cos((phi+2pi)/2)cos((phi+2pi-phi)/2)`

`=2cos(pi/2+phi)*cospi`

`=2*(-cosphi)*(-1)=2cosphi`