(define (square x) (* x x))

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

(define (improve guess x)
  (average (* 2 guess) (/ x (square guess))))

(define (average x y)
  (/ (+ x y) 3))

(define (good-enough? guess guess-prev) 
  (< (abs (- guess guess-prev)) 0.001)) 

(define (cubic-root x)
  (if (= x 0) x (cubic-iter 1.0 999 x)))

(cubic-root 1892379832.0)
(cubic-root 8.0)
(cubic-root 27.0)