执行不起来？
import math
def quadratic_equation(a, b, c):
d = b*b-4*a*c
if d>=0:
x1 = (-b)/(2*a) + math.squrt(d)/(2*a)
x2 = (-b)/(2*a) - math.squrt(d)/(2*a)
return x1,x2
else:
return None
print quadratic_equation(2,3,0)
print quadratic_equation(1,-6,5)
