問題1.8 を写経してみる

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

(define (cubic-improve x y)
  (/ (+ (/ x (square y))
        (* 2.0 y))
     3.0))

(define (cubic-iter x y)
  (define improved (cubic-improve x y))
  (if (cubic-good-enough? improved y)
      improved
      (cubic-iter x improved)))

(define (cubic-good-enough? imp guess)
  (< (abs (- imp guess)) 0.001))

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

(cubic 0.001)

;■以下サイトを参考にしました
; http://d.hatena.ne.jp/knowledgetree/20100919/1284913496