Mathematics
OpenStudy (anonymous):

What are the characteristics of a complex number?

OpenStudy (anonymous):

complex numbers are numbers that have both a real and non real part, using variables, a + bi, where i is the standard unit for an imaginary number, and a and b are real numbers

OpenStudy (anonymous):

the idea originated from the question of taking square roots of negative numbers. If you get your calculator out and try and do $\sqrt{-4}$ it won't like you. So mathematicians make the definition $\sqrt{-1} = i$ so one can answer problems like the above $\sqrt{-4} = \sqrt{-1}\sqrt{4} = 2i$