Answer:
Using the De Moivre's Theorem, let us work out for the fourth roots of 81(cos 320° + i sin 320° ).
zⁿ = rⁿ (cos nθ + i sin nθ)
z⁴ = 81(cos 320° + i sin 320° )
z = ∜[81(cos 320° + i sin 320° )]
= ∜[3^4 (cos 4*80° + i sin 4*80°)]
= 3(cos 80° + i sin 80°)
