A number in the form a + ib is called a complex number where a and b are natural

number and i is called an imaginary unit and i=√-1

We can solve the equations like x² - 9 = 0, x² - 4 = 0, etc. But we can not solve the

equation like x² + 1 = 0, x² + 4 = 0 etc. because there is no real number whose

square is a negative real number.  Euler was the first mathematician who had introduced

the symbol i(iota) with the property i² = -1 and i⁴ = 1

So by introducing i, now we can now solve the equations like x² + 1 = 0, x² + 4 = 0, etc.