公式：

# 牛顿法求三次根
def sqrt(n):
    k = n
    while abs(k*k-n)>1e-6:
        k = k - (k*k-n)/(2*k)
    print(k)

def cube_root(n):
    k = n
    while abs(k*k*k-n)>1e-6:
        k = k - (k*k*k-n)/(3*k*k)
    print(k)
sqrt(2)
cube_root(8)