SENa=√3/2 e SENb=1/2 calcule na adição de arcos: SENa(a+b) e COS(a+b)

I'm going to assume you mean sin(a) = √3/2 and sin(b) = 1/2.  If so then these refer to a right angled triangle with sides 1, √3 and 2.  Angle a is 60° and b is 30° so a+b = 90°. 

sin(90°) = 1 and cos(90°) = 0