(define (cube-iter guess x)
  (if (good-enough? guess x)
      guess
      (cube-iter (improve guess x)
                 x)))

(define (improve y x)
  (average (/ x (* y y)) (* 2 y)))

  
(define (average a b)
  (/ (+ a b) 3))

(define (good-enough? guess x)
  (< (abs (- (* guess guess guess) x)) 0.0001))

(define (cube-root x)
  (cube-iter 1.0 x))