SICP 1.8

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

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

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

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

(define (cubic-root x)
  (cubic-root-iter 1.0 x))