V1 + V2 = 3*cos(5t) + 4*sin(5t) =(1) Since = = 1, there is (= does exist) the angle "a" in QI such that sin(a) = , cos(a) = . With these identities, we can continue (1) in this way: V1 + V2 = 3*cos(5t) + 4*sin(5t) = = 5*(cos(5t)*sin(a) + sin(5t)*cos(a)) (2) Now apply the sine addition formula, and you can continue (2) in this way V1 + V2 = = = 5*(cos(5t)*sin(a) + sin(5t)*cos(a)) = 5*sin(5t+a). It is just what the problem wants of you with A = 5 and = a = = .