# Introduction to complex numbers

Contents

Learn about irrational numbers and how to identify them.

See how you score on these 18 practice questions

Learn about the imaginary unit i (which is the square root of -1) and about imaginary numbers like 3i (which is the square root of -9).

Learn about complex numbers (spoiler: they are numbers that consist of both real and imaginary parts).

Learn how we can visualize complex numbers in a plane. This can be seen as an expansion of the 1-dimensional real number line into a 2-dimensional plane!

Learn how to add or subtract complex numbers. For example, write (2+3i)-(1+2i) as (1+i).

Learn how to multiply complex numbers using the fact that i^2=-1 and the distributive property. For example, multiply (1+i) by (2+3i).