Decimal problem

[hmillsar]

Hello forum. I am currently writing a program to determine the roots of a cubic equation. However I have come across some trouble with my script.

import math
def sols(a, b, c, d ):
    disc = 18*a*b*c*d - 4*(b**3)*d + (b**2)*(c**2) - 4*a*(c**3) - 27*(a**2)*(d**2)
    x = b**2.0 - 3.0*a*c
    y = 2.0*(b**3.0) - 9.0*a*b*c + 27.0*(a**2.0)*d
    alpha = complex(-0.5, 0.5*math.sqrt(3))
    if disc >= 0:
        C = ((y + complex (0, math.sqrt (27.*(a**2)*disc)))/2)**(1.0/3)
        sol1 = -(1/3*a)*(b + C + x/C)
        sol2 = -(1/3*a)*(b + C*alpha + x/(C*alpha))
        sol3 = -(1/3*a)*(b + C*(alpha**2)) + x/(C*(alpha**2))

This is my program so far. The problem I have is that,  for the values a = 2, b = 3, c= -11, d = -6 :

In[11]: C

Out[12]: (7.5+4.330127018922192j)


In[13]: x/C

Out[14]: (7.500000000000002-4.330127018922193j)

In order to get sol1 I must add C and x/C. I want this to equal (15.00000000000002 + 0j)

Instead I get:

In[26]: C + x/C

Out[27]: (15.000000000000002-8.881784197001252e-16j)

I dont understand why I have gotten this result. I believe the complex parts should cancel each other. I am aware the last decimal point of C and x/C differ and i believe this is the problem. However the two complex parts should cancel, so I am not sure why I have this problem of the differing decimal value.

Anyone have any idea? 

Many thanks.

Also if you cannot already tell, I am very much a python newbie. Apologies if I have not made myself clear.