A complex number is a number that contains a real part and an
imaginary part. The imaginary part of a complex number contains the
imaginary unit, ı. This number is called imaginary because it is
equal to the square root of negative one. ı is not a real number.
Although it is rarely, if ever, used in some fields of math, it comes in very
handy when calculating the roots of polynomials, because the quadratics that
were previously irreducible over the reals are reducible over the complex
numbers. Before we get into that, though, we'll learn about the imaginary unit
ı, the complex plane, and a few operations involving complex numbers.
Then we'll be able to understand the true usefulness of complex numbers.