计算一个数的立方根

;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-advanced-reader.ss" "lang")((modname e18) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #t #t none #f ())))
(define (div-three x)
(/ x 3.0))

;;(div-three 1 3)

(define (three-times x)
(* x x x))

;;(three-times 2)

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

;;(square 2)

(define (make-nums guess x)
(+ (+ guess guess) (/ x (square guess))))

;;(make-nums 2 8)

(define (improve guess x)
(div-three (make-nums guess x))
)

;;(improve 2 8)

(define (good-enough? guess x)
(< (abs (- (three-times guess) x)) 0.001))

;;(good-enough? 2 8)

(define (three-sqrt-iter guess x )
(if (good-enough? guess x)
guess
(three-sqrt-iter (improve guess x) x)))

;;(three-sqrt-iter 2 8)

(define (three-sqrt x)
(three-sqrt-iter 1.0 x))

(three-sqrt 27)