(define (cbrt x)
  (cbrt-iter 1.0 x))

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

(define (improve guess x)
  (/
   (+ (/ x (* guess guess)) (* 2 guess))
   3))

(define (good-enough? guess x)
  (< (abs (- (* guess guess guess) x)) 0.001))