采用1.7中的變化率為終止檢測。
(define (cube-root x)
(cube-root-iter x 1.0 x))
(define (cube-root-iter last-guess guess x)
(if (enuf? last-guess guess)
guess
(cube-root-iter guess (improve guess x) x)))
(define (enuf? x y)
(< (/ (abs (- x y)) y) 0.001))
(define improve (lambda (y x)
(/ (+ (/ x (* y y)) (* 2 y)) 3)))