python函数

求一元二次方程的解：
import math
def quadratic (a,b,c):
    if a==0:
        return -c/b
    elif b*b-4*a*c>=0:
        x1=(-b+math.sqrt(b*b-4*a*c))/(2*a)
        x2=(-b-math.sqrt(b*b-4*a*c))/(2*a)
        return x1,x2
    else:
        return '无解'
r=quadratic(2,3,1)
print(r)
r=quadratic(0,2,4)
print(r)
r=quadratic(4,2,4)
print(r)