what is i

i can just be a pronumeral, like x.

or

i can stand for interest

or

i can be the sumbol of electrical current

or

i can be equal   square root of -1

\(i=\sqrt{-1}\\ \)

 

You cannot find the square root of any negative number - not in the real number system anyway.

 

\(\sqrt{-1}\;\;\text{is an imaginary number.}\\ \text{It is the basis of the complex number system}\)