Complex numbers were introduced in a prior lesson: Complex Numbers
Quickly read through this earlier lesson
and do plenty of the online exercises to make sure that you understand all the concepts there.
Knowledge of the complex numbers is needed to fully understand the Fundamental Theorem of Algebra
in an upcoming lesson.
For your convenience, this current lesson gives an inanutshell review of complex numbers and related concepts.
REVIEW
complex numbers and related concepts
Arithmetic with complex numbers is covered in the next section. 
a piece of the complex plane, showing the numbers $\,2 + i\,$ and $\,1 + 2i\,$ 
On this exercise, you will not key in your answer. However, you can check to see if your answer is correct. 
PROBLEM TYPES:
