Know there is a complex number i such that i^2 = -1, and every complex number has the form a + bi with a and b real.

What is the Imaginary Unit i?

Every tried to take the square root of a negative number? You'll need the imaginary unit 'i' to write the answer. This tutorial introduces you to this useful imaginary unit.
What is an Imaginary Number?

Imaginary numbers are used to help us work with numbers that involve taking the square root of a negative number. In this tutorial, you'll be introduced to imaginary numbers and learn that they're a type of complex number.