import math
def quadratic(a, b, c):
if a == 0:
return None
delta = b ** 2 - 4 * a * c
if delta < 0:
return None
delta = math.sqrt(delta)
x1 = (-1.0 * b + delta) / 2 / a
x2 = (-1.0 * b - delta) / 2 / a
return x1, x2
print(quadratic(1, 2, 3))
print(quadratic(1, 4, -4))