Use the concept of the phasor to combine the following sinusoidal functions into a single trigonometric expression:
a) y = 30 cos(200t - 160°) + 15 cos(200t + 70°),
b) y = 90 sin(50t - 20°) + 60 cos(200t - 70°),
c) y = 50 cos(5000t - 60°) + 25 sin(5000t + 110°) - 75 cos(5000t - 30°),
d) y = 10 cos (ωt + 30°) + 10 sin ωt + 10 cos(ωt + 150°).