Complex Numbers Definition
All the numbers we know are called real numbers and can be set on a straight line called the line of the real numbers. So numbers like 1, 2, -7, 4.78, -3.14, 2/5, the square root of positive numbers, are real numbers. They are set on the real numbers straight line and we can apply to them all the operations, like addition, multiplication, etc.
What if we want to calculate the square root of a negative number? There are no real numbers that could be the solution of the square root of a negative number. In this case the imaginary numbers are used.