PreCalculus - Trigonometry: Trig Identities (36 of 57) Product to Sum Formula 2

In this video I will prove cosAcosB=(1/2)[cos(A+B)+cos(A-B)] and sinAsinB=1/2)[cos(A+B)-cos(A-B)].

[sdgray]

I am horrible with trig identities, but you really saved me there. Very clear and easy to understand, thank you so much!!

[ChrisnaldoFabrolo]

Is the negative sign not supposed to change also when you divide cos(A+B)-cos(A-B) by 2? I mean are all the signs not supposed to change giving cos(A-B)+cos(A+B)? Why is the negative sign(-) in between cos(A+B) and cos(A-B) remain thesame after dividing by negative two(-2)?

[cesarluisnava]

That identity cosacosb can be proved using complex numbers?