In this series of posts, I explore properties of complex numbers that explain some surprising answers to exponential and logarithmic problems using a calculator (see video at the bottom of this post). These posts form the basis for a sequence of lectures given to my future secondary teachers.

To begin, we recall that the trigonometric form of a complex number is

where and , with in the appropriate quadrant. As noted before, this is analogous to converting from rectangular coordinates to polar coordinates.

Theorem. If , where and are real numbers, then

Definition. Let be a complex number so that . Then we define

.

Definition. Let and be complex numbers so that . Then we define

Technical point: for the latter two definitions, these are the principal values of the functions. In complex analysis, these are usually considered multiply-defined functions. But I’m not going to worry about this technicality here and will only consider the principal values.

In the remaining posts in this series, I want to explore which properties of exponential functions remain true when complex numbers are used.

To begin, if is a real rational number, then there is an alternative definition of that matches De Moivre’s Theorem. Happily, the two definitions agree. Suppose that with . Then

Next, one of the Laws of Exponents remains true even for complex numbers:

.

However, in previous posts, we’ve seen that the rules and may not be true if nonpositive bases, let alone complex bases, are used.

We can also derive the usual rules and . First,

.

Next, we think like an MIT freshman and use the above Law of Exponents to observe that

.

Dividing, we see that .

For completeness, here’s the movie that I use to engage my students when I begin this sequence of lectures.

I'm a Professor of Mathematics and a University Distinguished Teaching Professor at the University of North Texas. For eight years, I was co-director of Teach North Texas, UNT's program for preparing secondary teachers of mathematics and science.
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2 thoughts on “Calculators and complex numbers (Part 23)”

## 2 thoughts on “Calculators and complex numbers (Part 23)”