1 (define (cbrt x)
2   (cbrt-iter 1.0 x))
3 
4 (define (cbrt-iter guess x)
5   (if (good-enough? guess x)
6       guess
7       (cbrt-iter (improve guess x) x)))
8 
9 (define (good-enough? guess x)
10   (< (abs (- (cube guess) x)) 0.001))
11 
12 (define (improve guess x)
13   (/ (+ (/ x (square guess)) (* 2 guess)) 3))
14 (define (square x) (* x x))
15 (define (cube x) (* x x x))
16 
17 (cbrt 15)
18 ;; 2.46621207
19 (cbrt 8)
20 ;; 2
21 (cbrt 64)
22 ;; 4