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

(define (my-cube-rt x)
  (define (good-enough? guess prev-guess)
    (< (/ (abs (- guess prev-guess)) guess) 0.0001f))
  (define (improve guess)
    (/ (+ (/ x (square guess)) (* 2.0f guess)) 3.0f))
  (define (my-cube-rt-iter guess prev-guess)
    (if (good-enough? guess prev-guess)
      guess
      (my-cube-rt-iter (improve guess) guess)))
  (my-cube-rt-iter 2.0f 1.0f))

(display "Please enter a number: ")
(define a (read))
(display "You entered: ")
(display a)
(display "\nThe cubic-root of that is: ")
(display (my-cube-rt a))