# Number System - Imaginary Number

An imaginary number i is one solution to $x^2 = -1$ and is a part of the definition of a complex number

$$i = \sqrt { -1 }$$

It was invented because formulas sometimes required the manipulation of square roots of negative numbers.

Numbers such as i, -i, 3i, 2.17i are called imaginary number.

## Example

• Problem: $x^2 = -9$
• Solution: x = 3i