求立方根的牛顿法
(define (sqrt-iter guess x)
(if (good-enough? guess x)
guess
(sqrt-iter (improve guess x)
x)))
(define (improve guess x)
(/ (+ (/ x (* guess guess))(* 2 guess)) 3))
(define (good-enough? guess x)
(< (abs (- (improve guess x) guess))
.001))
(define (abs x)
(if (> x 0)
x
(- x)))
(define (square x)
(* x x))
(define (sqrt x)
(sqrt-iter 1.0 x))