import math

def quadratic(a, b, c):

    if not (isinstance(a,(int,float))&isinstance(b,(int,float))&isinstance(c,(int,float))):

        raise TypeError('Bad Operand Type!')

    delta=b*b-4*a*c;

    if(delta<0 or a==0):

        return None;

    else:

        x1=(-b+math.sqrt(delta))/(2*a)

        x2=(-b-math.sqrt(delta))/(2*a)

        return x1,x2

# 测试

print('quadratic(2, 3, 1) =', quadratic(2, 3, 1))

print('quadratic(1, 3, -4) =', quadratic(1, 3, -4))

if quadratic(2, 3, 1) != (-0.5, -1.0):

    print('测试失败')

elif quadratic(1, 3, -4) != (1.0, -4.0):

    print('测试失败')

else:

    print('测试成功')

print(quadratic(5.7,1,str));