# Complex Numbers

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Newton's Basin
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Simple Harmonic Motion

## Definition

Complex numbers are numbers which take the form $a+bi$, where $a$ and $b$ are real numbers and $i = \sqrt{-1}$ .

Such numbers frequently appear in mathematical equations, even in those describing physical systems. Extending our notion of numbers to include complex numbers results in many astounding symmetries and relationships throughout mathematics.

## Basic Properties

Complex numbers have two parts: a "real" part represented by $a$ and an "imaginary" part represented by $bi$; the $i$ factor in the imaginary part forces the two to be separate. The same operations that are used on real numbers, such as addition, subtraction, multiplication, and division, can be used on complex numbers. For example, two complex numbers are added by components, real added to real part and imaginary to imaginary part:

$(6-1i)+(3+0.5i) = 9 -0.5i\,$

As another example, multiplying two complex numbers is carried out in the same way that we would multiply two real binomials:

$(3+2i) \times (1-3i) = 3-9i+2i+6 = 9-7i \,$

Note that because each $i$ is the square root of $-1$, the product of two $i$ terms gives $-1$ , so $2i \times (-3i) = 6 \,$.

## Visualizing the Complex Numbers

We traditionally visualize the real numbers, such as 2 and 0.5, as points on the number line. We can visualize real numbers this way because all real numbers can be identified by a single value. The real number 5 is unique, and has its own place on the number line. Because complex numbers have two parts, we can think of them as vectors contained in a plane. We call the plane which contains complex numbers the Argand Plane, or the Complex Plane. The y-axis represents the imaginary component of our complex number, and the x-axis the real component. The complex number $2+3i$ is shown below:

We visualize complex numbers in the same way we visualize an ordered pair on a plane.

We can thus speak of the magnitude of a complex number as the length of this vector, which is $r = \sqrt{a^2+b^2}$, as is readily shown by the Pythagorean Theorem.

This vector idea leads to an important relation between trigonometry and the complex numbers. Euler's formula, which can be derived using Taylor Series, tells us that

$e^{i\theta} = \cos(\theta) +i\sin(\theta) \,$.
$\sin(\theta)=b/r \,$ and $\cos(\theta) = a/r \,$ .
Substituting gives
$e^{i\theta} = a/r +ib/r \,$
or
$re^{i\theta} = a+bi \,$.

Therefore all complex numbers of the form $a +bi$ can be expressed with an exponential function.