Imaginary unit

The imaginary unit, $i=\sqrt{-1}$ , is the fundamental component of all complex numbers. In fact, it is a complex number itself. It has a magnitude of 1, and can be written as $1 \text{cis } \left(\frac{\pi}{2}\right)$ . Any complex number can be expressed as $a+bi$ for some real numbers $a$ and $b$ .

Trigonometric function cis

Main article: cis

The trigonometric function $\text{cis } x$ is also defined as $e^{ix}$ or $\sin x+i(\cos x)$ .

Series

When $i$ is used in an exponential series, it repeats at every four terms:

$i^1=\sqrt{-1}$
$i^2=\sqrt{-1}\cdot\sqrt{-1}=-1$
$i^3=-1\cdot i=-i$
$i^4=-i\cdot i=-i^2=-(-1)=1$
$i^5=1\cdot i=i$

This has many useful properties.

Use in factorization

$i$ is often very helpful in factorization. For example, consider the difference of squares: $(a+b)(a-b)=a^2-b^2$ . With $i$ , it is possible to factor the otherwise-unfactorisable $a^2+b^2$ into $(a+bi)(a-bi)$ .

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Imaginary unit

Contents

Trigonometric function cis

Series

Use in factorization

Problems

Introductory

Intermediate

Olympiad

See also