Complex Analysis - Maths KU

20 Chapter 1 Complex Numbers and the Complex Plane
Note in Example 3, if we also want the value of z −3 , then we could proceed
in two ways: either find the reciprocal of z 3 = −8i or use (9) with n = −3.
de Moivre’s Formula When z = cos θ + i sin θ, wehave|z| = r =1,
and so (9) yields
(cos θ + i sin θ) n = cos nθ + i sin nθ. (10)
This last result is known as de Moivre’s formula and is useful in deriving
certain trigonometric identities involving cos nθ and sin nθ.See Problems 33
and 34 in Exercises 1.3.
EXAMPLE 4 de Moivre’s Formula
From (10), with θ = π/6, cos θ = √ 3/2 and sin θ =1/2:
�√ 3
2
1
+
2 i
�3 �
= cos 3θ + i sin 3θ = cos
= cos π π
+ i sin = i.
2 2
3 · π
6
Remarks Comparison with Real Analysis
� �
+ i sin
3 · π
6
(i) Observe in Example 2 that even though we used the principal arguments
of z1 and z2 that arg(z1/z2) =4π/3 �= Arg(z1/z2).Although
(8) is true for any arguments of z1 and z2, it is not true, in general,
that Arg(z1z2) =Arg(z1)+Arg(z2) and Arg(z1/z2) = Arg(z1)−
Arg(z2).See Problems 37 and 38 in Exercises 1.3.
(ii) An argument can be assigned to any nonzero complex number z.
However, for z = 0, arg(z) cannot be defined in any way that is
meaningful.
(iii) If we take arg(z) from the interval (−π, π), the relationship between
a complex number z and its argument is single-valued; that is, every
nonzero complex number has precisely one angle in (−π, π).But
there is nothing special about the interval (−π, π); we also establish
a single-valued relationship by using the interval (0, 2π) to define
the principal value of the argument of z.For the interval (−π, π),
the negative real axis is analogous to a barrier that we agree not to
cross; the technical name for this barrier is a branch cut.If we use
(0, 2π), the branch cut is the positive real axis.The concept of a
branch cut is important and will be examined in greater detail when
we study functions in Chapters 2 and 4.
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