Exercise 1.8

(define (square x) (* x x))
(define (cube x) (* x x x)) 
(define (cbrt-iter guess x) 
  (if (good-enough? guess x) 
      guess 
      (cbrt-iter (improve guess x) x))) 
(define (improve guess x) 
  (/ (+ (/ x (square guess)) 
        (* 2 guess)) 
     3)) 
(define (good-enough? guess x) 
  (< (abs (- (cube guess) x)) 0.001)) 
(define (cbrt x) 
  (cbrt-iter 1.0 x)) 

(cbrt 8) ~> 2.000004911675504