(define (cube-root x)
    (sqrt-iter 0.0 1.0 x)
)
(define (sqrt-iter prev guess x)
    (if (good-enough? prev guess)
        guess
        (sqrt-iter guess (improve guess x) x)
    )    
)
(define (good-enough? prev guess)
    (< (/ (abs (- guess prev)) guess) 0.001)
)
(define (improve guess x)
    (avg (/ x (* guess guess)) (* 2 guess))
)
(define (avg x y)
    (/ (+ x y) 3)
)