快速算根号

def f(s, k, x):
    a = k
    b = 1
    for i in range(x):
        t = a
        a = a * a + b * b * s
        b = 2 * t * b
    return a / b
math.sqrt(2)
1.4142135623730951
f(2, 1.5, 4)
1.4142135623730951