Python 3, 79 chars

<!-- language: lang-python -->

    a,b,c=eval(input())
    d=(b*b-4*a*c)**.5/2/a
    x=-b/2/a
    print('r1=',x+d,'\nr2=',x-d)

Python's imaginary unity is `j` and not `i`. I used Python 3, because the power operator works also on negative numbers.


BTW, is it really needed to write `r1= ...` and `r2= ...` ?