why is i the square root of -1

Question

i understand that mathematicians use 'i' to simplify 'the square root of -1'. Why is that?

Answer 1

Because it's simpler than writing √(-1) all the time.  i is the first letter in "imaginary" and is the unit imaginary number.  It is "imaginary" because it is not "real"; that is, there is no real number that is √(-1).  We use pi and e to represent other special numbers; i is just a special number like pi and e.