The process of **mathematical induction** can be used to prove a very important theorem in mathematics known as **De Moivre's theorem**. If the complex number *z = r*(cos α + *i* sin α), then

The preceding pattern can be extended, using mathematical induction, to De Moivre's theorem.

If *z = r*(cos α + *i* sin α), and *n* is a natural number, then

**Example 1:** Write *s + bi*.

First determine the radius:

Since cos α =

**Example 2:** Write *a + bi*.

First determine the radius:

Since cos

Problems involving powers of complex numbers can be solved using binomial expansion, but applying De Moivre's theorem is usually more direct.

De Moivre's theorem can be extended to roots of complex numbers yielding the **nth root theorem**. Given a complex number *z = r*(cos α + *i* sinα), all of the *n*th roots of *z* are given by

where *k* = 0, 1, 2, …, (n − 1)

If *k* = 0, this formula reduces to

This root is known as the **principal nth root** of *z*. If α = 0° and *r* = 1, then *z* = 1 and the **nth roots of unity** are given by

where *k* = 0, 1, 2, …, ( *n* − 1)

**Example 3:** What are each of the five fifth‐roots of

Since cos

and applying the *n*th root theorem, the five fifth‐roots of *z* are given by

where *k* = 0, 1, 2, 3, and 4

Thus the five fifth‐roots are

Observe the even spacing of the five roots around the circle in Figure 1 .

**Figure 1**

Drawing for Example 3.